Support dropping non-image files onto markdown editor as attachments

When dragging files like .zip onto the issue description editor, non-image
files are now uploaded as attachments instead of being silently ignored.
Images continue to be handled inline by MDXEditor's image plugin.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/InlineEditor.tsx

@@ -11,6 +11,8 @@ interface InlineEditorProps {
   placeholder?: string;
   multiline?: boolean;
   imageUploadHandler?: (file: File) => Promise<string>;
+  /** Called when a non-image file is dropped onto the editor. */
+  onDropFile?: (file: File) => Promise<void>;
   mentions?: MentionOption[];
   nullable?: boolean;
 }
@@ -46,6 +48,7 @@ export function InlineEditor({
   multiline = false,
   nullable = false,
   imageUploadHandler,
+  onDropFile,
   mentions,
 }: InlineEditorProps) {
   const [editing, setEditing] = useState(false);
@@ -228,6 +231,7 @@ export function InlineEditor({
           className="bg-transparent"
           contentClassName={cn("paperclip-edit-in-place-content", className)}
           imageUploadHandler={imageUploadHandler}
+          onDropFile={onDropFile}
           mentions={mentions}
           onSubmit={() => {
             finalizeMultilineBlurOrSubmit();