Mobile: opening an attachment names the file after the link text instead of the imeta filename #8065

Summary

On mobile, tapping a file attachment link downloads the file and opens it under the link text instead of the attachment's imeta filename. When the label is prose rather than a file name, the opened or saved file loses its real name and extension.

Where

mobile/lib/features/channels/message_content.dart, in _buildLink: the authenticated download is opened with the link text as the file name:

await ref.read(openDownloadedFileProvider)(
  url,
  auth.headersFor(url),
  text,
);

_buildLink already receives the matching ImetaEntry? imeta, and ImetaEntry (mobile/lib/features/channels/message_media.dart) parses filename. It just isn't used here.

Steps to reproduce

1. Send a message with a non-media attachment whose link label is not the file name, e.g. content [Quarterly report](https://<relay>/media/<sha256>.pdf) with imeta containing filename q3-report.pdf.
2. Tap the link on mobile.

Expected: the file opens as q3-report.pdf.
Actual: it is opened under Quarterly report, without the .pdf extension, so the system viewer may not recognize it.

Suggested fix

Prefer the imeta filename and fall back to the link text:

imeta?.filename ?? text,

A widget test next to opens local file links through an authenticated download in mobile/test/features/channels/message_content_test.dart can pin it, with a labelled link plus an imeta tag carrying filename.