extensions: Check the extension of name not path on drag and drop

Update existing code as a preparation to get drag-and-drop work on
desktop android, where the path being the content URI is not guaranteed
to have the same suffix as the file name displayed to the user. This
change itself doesn't add android support.

Bug: 392777363
Test: drag-and-dropping a .crx file works on linux
Change-Id: Id98dd1afb2bffbd9538ab191f10b8deee7392afd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6380614
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Keigo Oka <oka@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1437212}