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While not a very strong security boundary, making it possible for
users to know what extension a file will be saved with is a good idea.

This also fixes support for compound extensions with the File System
Access API (i.e. ".tar.gz"). The mac file dialog already had a
workaround if the default path ended in such an extension, but the
same problem occurs if the file type filters include a type with a
compound extension.

Bug: 1137247
Change-Id: I492bf36baced3de044b8fed5d57fc7b9b5b64400
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2582842Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835803}