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/pages/IssueDetail.tsx

@@ -1329,6 +1329,9 @@ export function IssueDetail() {
             const attachment = await uploadAttachment.mutateAsync(file);
             return attachment.contentPath;
           }}
+          onDropFile={async (file) => {
+            await uploadAttachment.mutateAsync(file);
+          }}
         />
       </div>