Prevent default browser behavior with Plupload disallowed file extensions

[nacin]

See #22149. When Plupload is set to accept only certain file types, the drag event falls back to the browser's default behavior. That means HTML files replace the entire viewport, ZIP files get re-downloaded, etc. Ideally, our own handlers intercept such a drag event and prevents anything else from happening.

[antpb]

We could add some jQuery to the admin-header file to disallow drag and drop events. The following patch fixes it. I've tested the media upload page's drag and drop feature after applying the patch and this does not overwrite the functionality.

[azaozz]

In 35019:

Plupload: update to 2.1.8. Changelog: ​https://github.com/moxiecode/plupload/releases.

Hide the editor uploader overlay on all drop events in the window, not only when dropping on the editor uploader element.

Fixes #34251, #22403.

[azaozz]

Fixed by updating to Plupload 2.1.8. It properly fires the error and prevents the default event when a file extension is rejected.