Extract and use charset to properly serve inline text attachments (#23432)

* Extract and use charset to properly encode attachments

* Add the content type for external URLs

* Be more cautious when parsing charset from `file`

app/contracts/attachments/create_contract.rb

@@ -36,6 +36,7 @@ module Attachments
     attribute :digest
     attribute :description
     attribute :content_type
+    attribute :charset
     attribute :container
     attribute :container_type
     attribute :author

app/models/attachment.rb

@@ -93,7 +93,9 @@ class Attachment < ApplicationRecord
   # specifically when using S3 for attachments. In the case of S3 the file name for the downloaded
   # file will still be correct as it's part of the URL before the query.
   def external_url_options(expires_in: nil)
-    { content_disposition: content_disposition(include_filename: false), expires_in: }
+    { content_disposition: content_disposition(include_filename: false),
+      content_type: served_content_type,
+      expires_in: }
   end
 
   def external_storage?
@@ -119,6 +121,30 @@ class Attachment < ApplicationRecord
     end
   end
 
+  # Returns the Content-Type to use when serving this file inline in a browser.
+  # Text files are normalised to text/plain (prevents script execution) with an
+  # explicit charset. Non-inlineable files get application/octet-stream so the
+  # browser is forced to download them.
+  def served_content_type
+    if is_text?
+      "text/plain; charset=#{charset.presence || Setting.attachment_default_charset}"
+    elsif inlineable?
+      content_type
+    else
+      "application/octet-stream"
+    end
+  end
+
+  # Returns the content type to use when serving the file to a browser.
+  # For text files, ensures a charset is always present so browsers don't
+  # fall back to ISO-8859-1. Preserves the real MIME subtype (e.g. text/x-ruby)
+  # unlike served_content_type which normalises to text/plain for security.
+  def serving_content_type
+    return content_type unless is_text?
+
+    "#{content_type}; charset=#{charset.presence || Setting.attachment_default_charset}"
+  end
+
   def visible?(user = User.current)
     allowed_or_author?(user) do
       container.attachments_visible?(user)
@@ -227,7 +253,7 @@ class Attachment < ApplicationRecord
   end
 
   def set_content_type(file)
-    self.content_type = self.class.content_type_for(file.path)
+    self.content_type, self.charset = OpenProject::ContentTypeDetector.new(file.path).detect_with_charset
   end
 
   def set_digest(file)

app/uploaders/fog_file_uploader.rb

@@ -74,6 +74,7 @@ class FogFileUploader < CarrierWave::Uploader::Base
   #
   # @param options [Hash] Options hash.
   # @option options [String] :content_disposition Pass this content disposition to S3 so that it serves the file with it.
+  # @option options [String] :content_type Pass this content type to S3 so that it serves the file with it.
   # @option options [DateTime] :expires_at Date at which the link should expire (default: now + 5 minutes)
   # @option options [ActiveSupport::Duration] :expires_in Duration in which the link should expire.
   #
@@ -82,6 +83,7 @@ class FogFileUploader < CarrierWave::Uploader::Base
     url_options = {}
 
     set_content_disposition!(url_options, options:)
+    set_content_type!(url_options, options:)
     set_expires_at!(url_options, options:)
 
     remote_file.url url_options
@@ -103,15 +105,17 @@ class FogFileUploader < CarrierWave::Uploader::Base
   private
 
   def set_content_disposition!(url_options, options:)
-    if options[:content_disposition].present?
+    return if options[:content_disposition].blank?
-      url_options[:query] = {
+
-        # Passing this option to S3 will make it serve the file with the
+    (url_options[:query] ||= {})["response-content-disposition"] = options[:content_disposition]
-        # respective content disposition. Without it no content disposition
+  end
-        # header is sent. This only works for S3 but we don't support
+
-        # anything else anyway (see carrierwave.rb).
+  def set_content_type!(url_options, options:)
-        "response-content-disposition" => options[:content_disposition]
+    return if options[:content_type].blank?
-      }
+
-    end
+    # Like the content disposition above, this makes S3 serve the file with the
+    # given Content-Type, overriding the stored object type.
+    (url_options[:query] ||= {})["response-content-type"] = options[:content_type]
   end
 
   def set_expires_at!(url_options, options:)

config/constants/settings/definition.rb

@@ -81,6 +81,11 @@ module Settings
       organization_name: {
         default: "My Organization"
       },
+      attachment_default_charset: {
+        description: "Fallback charset used when serving text attachments whose encoding was not detected on upload",
+        format: :string,
+        default: "utf-8"
+      },
       attachment_max_size: {
         default: 5120
       },

config/locales/en.yml

@@ -1808,6 +1808,7 @@ en:
       attachment:
         attachment_content: "Attachment content"
         attachment_file_name: "Attachment file name"
+        charset: "Character set"
         content_type: "Content-type"
         downloads: "Downloads"
         file: "File"

db/migrate/20260528120000_add_charset_to_attachments.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+class AddCharsetToAttachments < ActiveRecord::Migration[8.0]
+  def change
+    add_column :attachments, :charset, :text
+  end
+end

lib/api/helpers/attachment_renderer.rb

@@ -100,16 +100,7 @@ module API
       end
 
       def attachment_content_type(attachment)
-        if attachment.is_text?
+        attachment.served_content_type
-          # Even if the text mime type might differ, always output plain text
-          # so this doesn't get interpreted as e.g., a script or html file
-          "text/plain"
-        elsif attachment.inlineable?
-          attachment.content_type
-        else
-          # For security reasons, mark all non-inlinable files as an octet-stream first
-          "application/octet-stream"
-        end
       end
 
       def set_cache_headers

lib/open_project/content_type_detector.rb

@@ -92,19 +92,19 @@ module OpenProject
       @filename = filename
     end
 
-    # Returns a String describing the file's content type
+    # Returns [mime_type, charset_or_nil], running the file command once.
-    def detect
+    def detect_with_charset
-      if blank_name?
+      return [SENSIBLE_DEFAULT, nil] if blank_name?
-        SENSIBLE_DEFAULT
+      return [EMPTY_TYPE, nil] if empty_file?
-      elsif empty_file?
+
-        EMPTY_TYPE
+      raw_mime, charset = FileCommandContentTypeDetector.new(@filename).detect
-      elsif calculated_type_matches.any?
+      [resolve_mime(raw_mime), charset]
-        calculated_type_matches.first
-      else
-        type_from_file_command || SENSIBLE_DEFAULT
-      end.to_s
     end
 
+    # Detecting only the mime type is effectively the
+    # first argument of +detect_with_charset+
+    def detect = detect_with_charset.first
+
     private
 
     def empty_file?
@@ -121,12 +121,9 @@ module OpenProject
       MIME::Types.type_for(@filename).map(&:content_type)
     end
 
-    def calculated_type_matches
+    def resolve_mime(raw_mime)
-      possible_types.select { |content_type| content_type == type_from_file_command }
+      matches = possible_types.select { |ct| ct == raw_mime }
-    end
+      (matches.first || raw_mime || SENSIBLE_DEFAULT).to_s
-
-    def type_from_file_command
-      @type_from_file_command ||= FileCommandContentTypeDetector.new(@filename).detect
     end
   end
 end

lib/open_project/file_command_content_type_detector.rb

@@ -69,23 +69,34 @@ module OpenProject
       @filename = filename
     end
 
+    # Returns [mime_type, charset_or_nil], e.g.:
+    #   ["text/plain", "utf-8"]
+    #   ["image/png",  nil]
     def detect
-      type_from_file_command
+      @detect ||= parse_file_command
     end
 
     private
 
-    def type_from_file_command
+    def parse_file_command
       # On BSDs, `file` doesn't give a result code of 1 if the file doesn't exist.
       type, status = Open3.capture2("file", "-b", "--mime", "--", @filename)
+      return [SENSIBLE_DEFAULT, nil] if type.nil? || status.to_i > 0 || type.match(/\(.*?\)/)
 
-      if type.nil? || status.to_i > 0 || type.match(/\(.*?\)/)
+      extract_mime_and_charset(type.strip)
-        type = SENSIBLE_DEFAULT
-      end
-      type.split(/[:;\s]+/)[0]
     rescue StandardError => e
       Rails.logger.info { "Failed to get mime type from #{@filename}: #{e} #{e.message}" }
-      SENSIBLE_DEFAULT
+      [SENSIBLE_DEFAULT, nil]
+    end
+
+    def extract_mime_and_charset(type)
+      parts = type.split(";").map(&:strip)
+      mime = parts.first
+      charset = parts.drop(1)
+                     .filter_map { |p| p.match(/\Acharset=([^\s;]+)\z/)&.[](1) }
+                     .first
+      charset = nil if charset == "binary"
+      [mime, charset]
     end
   end
 end

spec/fixtures/encoding/utf-8.txt

@@ -0,0 +1 @@
+UTF-8 encoded text with non-ASCII characters: äöü €

spec/lib/open_project/content_type_detector_spec.rb

@@ -90,7 +90,7 @@ RSpec.describe OpenProject::ContentTypeDetector do
     File.open(@filename, "w+") do |file|
       file.puts "This is a text file."
       file.rewind
-      expect(OpenProject::ContentTypeDetector.new(file.path).detect).to eq("text/plain")
+      expect(described_class.new(file.path).detect).to start_with("text/plain")
     end
     FileUtils.rm @filename
   end

spec/lib/open_project/file_command_content_type_detector_spec.rb

@@ -59,35 +59,36 @@
 require "spec_helper"
 
 RSpec.describe OpenProject::FileCommandContentTypeDetector do
-  it "returns a content type based on the content of the file" do
+  it "returns a [mime_type, charset] tuple for a text file" do
     tempfile = Tempfile.new("something")
     tempfile.write("This is a file.")
     tempfile.rewind
 
-    expect(described_class.new(tempfile.path).detect).to eq("text/plain")
+    mime, charset = described_class.new(tempfile.path).detect
+    expect(mime).to eq("text/plain")
+    expect(charset).to be_a(String)
 
     tempfile.close
   end
 
-  it "returns a sensible default when the file command is missing" do
+  it "returns [sensible_default, nil] when the file command is missing" do
     allow(Open3).to receive(:capture2).and_raise "o noes!"
-    filename = "/path/to/something"
+    expect(described_class.new("/path/to/something").detect).to eq(["application/binary", nil])
-    expect(described_class.new(filename).detect).to eq("application/binary")
   end
 
-  it "returns a sensible default on the odd chance that run returns nil" do
+  it "returns [sensible_default, nil] on the odd chance that run returns nil" do
     allow(Open3).to receive(:capture2).and_return [nil, 0]
-    expect(described_class.new("windows").detect).to eq("application/binary")
+    expect(described_class.new("windows").detect).to eq(["application/binary", nil])
   end
 
-  it "returns a sensible default when the file command returns an error code" do
+  it "returns [sensible_default, nil] when the file command returns an error code" do
     allow(Open3).to receive(:capture2).and_return ["text/plain", 1]
-    expect(described_class.new("windows").detect).to eq("application/binary")
+    expect(described_class.new("windows").detect).to eq(["application/binary", nil])
   end
 
-  it "returns a sensible default when the file command returns a type with parentheses" do
+  it "returns [sensible_default, nil] when the file command returns a type with parentheses" do
     allow(Open3).to receive(:capture2).and_return ["text/plain (with something)", 0]
-    expect(described_class.new("windows").detect).to eq("application/binary")
+    expect(described_class.new("windows").detect).to eq(["application/binary", nil])
   end
 
   it "uses end-of-input delimiter to prevent command injection" do
@@ -97,4 +98,47 @@ RSpec.describe OpenProject::FileCommandContentTypeDetector do
 
     expect(Open3).to have_received(:capture2).with("file", "-b", "--mime", "--", "--help")
   end
+
+  describe "charset parsing edge cases" do
+    def detect(raw_output)
+      allow(Open3).to receive(:capture2).and_return [raw_output, 0]
+      described_class.new("any").detect
+    end
+
+    it "extracts charset when followed by an extra unknown parameter" do
+      expect(detect("text/plain; charset=utf-8; taste=banana")).to eq(["text/plain", "utf-8"])
+    end
+
+    it "only captures the charset token, not trailing content" do
+      expect(detect("text/plain; charset=utf-8;extra")).to eq(["text/plain", "utf-8"])
+    end
+
+    it "returns nil charset when charset= has no value" do
+      expect(detect("text/plain; charset=")).to eq(["text/plain", nil])
+    end
+
+    it "ignores unrecognised parameters before charset" do
+      expect(detect("text/plain; taste=banana; charset=iso-8859-1")).to eq(["text/plain", "iso-8859-1"])
+    end
+  end
+
+  describe "charset detection from real fixture files" do
+    let(:utf8_fixture)    { Rails.root.join("spec/fixtures/encoding/utf-8.txt").to_s }
+    let(:iso8859_fixture) { Rails.root.join("spec/fixtures/encoding/iso-8859-1.txt").to_s }
+    let(:png_fixture)     { Rails.root.join("spec/fixtures/files/image.png").to_s }
+
+    it "detects utf-8 charset for a UTF-8 encoded file" do
+      expect(described_class.new(utf8_fixture).detect).to eq(["text/plain", "utf-8"])
+    end
+
+    it "detects iso-8859-1 charset for an ISO-8859-1 encoded file" do
+      expect(described_class.new(iso8859_fixture).detect).to eq(["text/plain", "iso-8859-1"])
+    end
+
+    it "returns nil charset for non-text files" do
+      mime, charset = described_class.new(png_fixture).detect
+      expect(mime).to eq("image/png")
+      expect(charset).to be_nil
+    end
+  end
 end

spec/models/attachment_spec.rb

@@ -252,7 +252,7 @@ RSpec.describe Attachment do
     let(:author) { create(:user) }
 
     let(:image_path) { Rails.root.join("spec/fixtures/files/image.png") }
-    let(:text_path) { Rails.root.join("spec/fixtures/files/testfile.txt") }
+    let(:text_path) { Rails.root.join("spec/fixtures/files/testfile-utf8.txt") }
     let(:binary_path) { Rails.root.join("spec/fixtures/files/textfile.txt.gz") }
 
     let(:image_attachment) { FogAttachment.new author:, file: File.open(image_path) }
@@ -320,6 +320,11 @@ RSpec.describe Attachment do
       it_behaves_like "it uses content disposition inline" do
         let(:attachment) { text_attachment }
       end
+
+      it "includes response-content-type with text/plain and the detected charset in the S3 URL" do
+        url = text_attachment.external_url.to_s
+        expect(url).to include "response-content-type=text%2Fplain%3B%20charset%3D"
+      end
     end
 
     describe "for a video file" do
@@ -351,6 +356,58 @@ RSpec.describe Attachment do
         expect(binary_attachment.content_disposition).to eq "attachment; filename=textfile.txt.gz"
         expect(binary_attachment.external_url.to_s).to include "response-content-disposition=attachment"
       end
+
+      it "includes response-content-type application/octet-stream in the S3 URL" do
+        expect(binary_attachment.external_url.to_s).to include "response-content-type=application%2Foctet-stream"
+      end
+    end
+  end
+
+  describe "#serving_content_type" do
+    subject(:attachment) { described_class.new(content_type:, charset:) }
+
+    let(:charset) { nil }
+
+    context "when a text file has a detected utf-8 charset (new upload)" do
+      let(:content_type) { "text/plain" }
+      let(:charset) { "utf-8" }
+
+      it "combines content_type and charset" do
+        expect(attachment.serving_content_type).to eq("text/plain; charset=utf-8")
+      end
+    end
+
+    context "when a text file has a non-UTF-8 charset (e.g. ISO-8859-1)" do
+      let(:content_type) { "text/plain" }
+      let(:charset) { "iso-8859-1" }
+
+      it "uses the stored charset" do
+        expect(attachment.serving_content_type).to eq("text/plain; charset=iso-8859-1")
+      end
+    end
+
+    context "when a text file has no charset stored (legacy upload)" do
+      let(:content_type) { "text/plain" }
+
+      it "falls back to Setting.attachment_default_charset so browsers do not default to ISO-8859-1" do
+        expect(attachment.serving_content_type).to eq("text/plain; charset=#{Setting.attachment_default_charset}")
+      end
+    end
+
+    context "when another text subtype has no charset stored" do
+      let(:content_type) { "text/x-ruby" }
+
+      it "falls back to Setting.attachment_default_charset" do
+        expect(attachment.serving_content_type).to eq("text/x-ruby; charset=#{Setting.attachment_default_charset}")
+      end
+    end
+
+    context "when the file is not a text type" do
+      let(:content_type) { "image/png" }
+
+      it "returns the content type unchanged" do
+        expect(attachment.serving_content_type).to eq("image/png")
+      end
     end
   end
 

spec/requests/api/v3/attachments/attachment_resource_shared_examples.rb

@@ -486,19 +486,75 @@ RSpec.shared_examples "an APIv3 attachment resource", content_type: :json, type:
         end
       end
 
-      context "for a local text file" do
+      context "for a local text file (no stored charset, uses configured default)" do
         it_behaves_like "for a local file" do
-          let(:expected_content_type) { "text/plain" }
+          let(:expected_content_type) { "text/plain; charset=#{Setting.attachment_default_charset}" }
           let(:mock_file) { FileHelpers.mock_uploaded_file name: "foobar.txt" }
           let(:content_disposition) { "inline; filename=foobar.txt" }
+          let(:attachment) do
+            att = create(:attachment, container:, file: mock_file, author: current_user)
+            att.file.store!
+            att.send :write_attribute, :file, mock_file.original_filename
+            att.send :write_attribute, :content_type, "text/plain"
+            att.send :write_attribute, :charset, nil
+            att.save!
+            att
+          end
         end
       end
 
-      context "for a local JS file" do
+      context "for a local JS file (normalised to text/plain, uses configured default charset)" do
         it_behaves_like "for a local file" do
-          let(:expected_content_type) { "text/plain" }
+          let(:expected_content_type) { "text/plain; charset=#{Setting.attachment_default_charset}" }
           let(:mock_file) { FileHelpers.mock_uploaded_file name: "foobar.js", content_type: "text/x-javascript" }
           let(:content_disposition) { "inline; filename=foobar.js" }
+          let(:attachment) do
+            att = create(:attachment, container:, file: mock_file, author: current_user)
+            att.file.store!
+            att.send :write_attribute, :file, mock_file.original_filename
+            att.send :write_attribute, :content_type, "text/x-javascript"
+            att.send :write_attribute, :charset, nil
+            att.save!
+            att
+          end
+        end
+      end
+
+      context "for a local UTF-8 text file" do
+        it_behaves_like "for a local file" do
+          let(:expected_content_type) { "text/plain; charset=utf-8" }
+          let(:mock_file) { FileHelpers.mock_uploaded_file name: "foobar.txt" }
+          let(:content_disposition) { "inline; filename=foobar.txt" }
+          let(:attachment) do
+            att = create(:attachment, container:, file: mock_file, author: current_user)
+            att.file.store!
+            att.send :write_attribute, :file, mock_file.original_filename
+            att.send :write_attribute, :content_type, "text/plain"
+            att.send :write_attribute, :charset, "utf-8"
+            att.save!
+            att
+          end
+        end
+      end
+
+      context "for a local ISO-8859-1 text file" do
+        it_behaves_like "for a local file" do
+          let(:expected_content_type) { "text/plain; charset=iso-8859-1" }
+          let(:mock_file) do
+            FileHelpers.mock_uploaded_file name: "iso.txt",
+                                           content: Rails.root.join("spec/fixtures/encoding/iso-8859-1.txt").binread,
+                                           binary: true
+          end
+          let(:content_disposition) { "inline; filename=iso.txt" }
+          let(:attachment) do
+            att = create(:attachment, container:, file: mock_file, author: current_user)
+            att.file.store!
+            att.send :write_attribute, :file, mock_file.original_filename
+            att.send :write_attribute, :content_type, "text/plain"
+            att.send :write_attribute, :charset, "iso-8859-1"
+            att.save!
+            att
+          end
         end
       end