Customer chat: rejected MIME types are accepted when the filename extension matches

[marassteiner]

Follow-up from #1297 (not a merge blocker).

Evidence

src/screens/chat.screen.tsx:44 (isAcceptedAttachment): if file.type is present but not in ACCEPTED_MIME_TYPES, the function still returns true when the name ends with .pdf / .jpeg / .jpg / .png.

A file named payload.png with type: image/svg+xml or text/html is therefore accepted. isImageFile (line 51) then treats any image/* MIME as a preview candidate.

The composer preview sink itself is still only a blob: object URL (asBlobPreviewUrl at line 60), so this is a type-policy gap, not an HTML injection path.

Suggested fix

Fall back to the extension only when file.type is empty. If a MIME type is present, it must be in the allow-list. Restrict isImageFile to image/jpeg, image/jpg, and image/png.