fix: use thumbnail only if present

[krille-chan]

No description provided.

[cursor]

PR Summary

Low Risk
Localized change to returned MatrixFile metadata in one code path; reduces mislabeled downloads without altering download/decrypt logic.

Overview
Fixes incorrect filename and MIME type on MatrixFile results from downloadAndDecryptAttachment when callers ask for a thumbnail but the event has none.

Introduces useThumbnail = getThumbnail && hasThumbnail so returned metadata only uses thumbnail naming (*.thumbnail.<ext>) and thumbnailMimetype when a thumbnail actually exists; otherwise the full attachment’s name and attachmentMimetype are used. This matches cases where download logic may still resolve the main attachment bytes when no thumbnail URL is present.

^{Reviewed by Cursor Bugbot for commit 7cdd7fb. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.29%. Comparing base (c8f1062) to head (7cdd7fb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2395   +/-   ##
=======================================
  Coverage   59.29%   59.29%           
=======================================
  Files         161      161           
  Lines       20303    20303           
=======================================
  Hits        12038    12038           
  Misses       8265     8265           

Files with missing lines	Coverage Δ
lib/src/event.dart	87.90% <100.00%> (ø)

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[td-famedly]

sure