We want to remove completely our custom modal for uploading files in composer and directly trigger the system file picker.
This PR makes it happen. The fix is pretty simple since we already weren't using our custom modal on mobile. We just need to start using the same hidden <input type="file"> that we already use on mobile.
It seems to be pretty tricky to test opening a system modal so I haven't added new tests. We already have other tests for file uploading though. We directly trigger jquery-File-Upload plugin hooks in those tests - 3dda926cb2/app/assets/javascripts/discourse/tests/acceptance/composer-attachment-test.js (L89).
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