fix(file-uploader): align accept parsing with native file input

[nehaaprasaad]

Closes #22111

Fixes multi-value accept parsing in file uploader components.

Changelog

New
None.

Changed
Parse accept with spaces and commas.
Normalize native input accept to comma-separated tokens.
Removed

None.

Testing / Reviewing

Test accept="image/jpeg image/png".
Test accept="image/jpeg,image/png".
Confirm unsupported files are not added.
Confirm single-value and extension-only still work.

PR Checklist

• As the author of this PR, before marking ready for review, confirm you:
  [ ] Reviewed every line of the diff
  [ ] Updated documentation and storybook examples
  [ ] Wrote passing tests that cover this change
  [ ] Addressed any impact on accessibility (a11y)
  [ ] Tested for cross-browser consistency
  [ ] Validated that this code is ready for review and status checks should pass

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[tay1orjones]

Please update to use the pull request template.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.07%. Comparing base (cdca471) to head (b4e93a9).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #22134      +/-   ##
==========================================
- Coverage   95.09%   95.07%   -0.03%     
==========================================
  Files         541      541              
  Lines       45179    45179              
  Branches     6537     6482      -55     
==========================================
- Hits        42965    42954      -11     
- Misses       2085     2096      +11     
  Partials      129      129              

Flag	Coverage Δ
main-packages	89.16% <ø> (ø)
web-components	97.87% <ø> (ø)

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[nehaaprasaad]

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