fix: reject suspicious filenames and match file icons case-insensitively

- Short-circuit validateAndDetectFileType when validateFilename flags a
  suspicious name so files like foo.exe.md cannot bypass the attachment
  allowlist via the new extension MIME fallback.
- Lowercase fileType in getFileIcon so uppercase extensions (e.g. .MD,
  .PDF) resolve to the correct icon.

apps/web/core/components/icons/attachment/attachment-icon.tsx

@@ -25,7 +25,7 @@ import {
 } from "@/components/icons/attachment";
 
 export const getFileIcon = (fileType: string, size: number = 28) => {
-  switch (fileType) {
+  switch (fileType.toLowerCase()) {
     case "pdf":
       return <PdfIcon height={size} width={size} />;
     case "csv":

packages/services/src/file/helper.ts

@@ -114,6 +114,7 @@ const validateAndDetectFileType = async (file: File): Promise<string> => {
   const filenameError = validateFilename(file.name);
   if (filenameError) {
     console.warn(`File validation warning: ${filenameError}`);
+    return "";
   }
 
   try {