Using HTML 5 File API together with a regular <input type=file /> field. This won't help the IE8s of the world, but will help users of non-standard, but modern operating systems and browsers.
This is not necessarily a complete proper patch, but it's worth reviewing in my opinion. Problems:
- PhabricatorFileUploadDialogController is unused now and I'm scared to try deleting it.
- This probably doesn't play very well with multiple Remarkup textareas on the page.
- There is no way to discover drag&drop and copy&paste uploads now, without the warning dialog.