Name the file in the remove button's failure description

The remove button's label is "Remove attachment" and doesn't say which
file. Its failure description now reads "<file>: <reason>", the same text
as the tooltip.

frontend/src/components/message-input/AttachmentChipList.test.tsx

@@ -106,8 +106,9 @@ describe('AttachmentChipList failed chip', () => {
     )!;
     const describedBy = remove.getAttribute('aria-describedby');
     expect(describedBy).toBeTruthy();
+    // Named, since the button's own label doesn't say which file.
     expect(document.getElementById(describedBy!)?.textContent).toBe(
-      'Upload failed. The file could not be read.',
+      'scan.pdf: Upload failed. The file could not be read.',
     );
   });
 

frontend/src/components/message-input/AttachmentChipList.tsx

@@ -124,7 +124,7 @@ export default function AttachmentChipList({
               </span>
               {failureReason && (
                 <span id={reasonId} hidden>
-                  {failureReason}
+                  {attachment.fileName}: {failureReason}
                 </span>
               )}