fix(filepicker): Round previews in filepicker dialog

[jancborchardt]

Add our standard border-radius, just like in the file list in Files. :)

Before	After

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.79%. Comparing base (d0feeab) to head (c21f65d).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2397   +/-   ##
=======================================
  Coverage   43.79%   43.79%           
=======================================
  Files          15       15           
  Lines         427      427           
  Branches       81       81           
=======================================
  Hits          187      187           
  Misses        211      211           
  Partials       29       29           

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