Improvements of Image format dropzone

* Limit drag-drop Image format zone to HTML5 runtime
* Only keep the first file dropped
* Only upload image files
* Fix CSS for the progress bar

props azaozz. see #24291.

git-svn-id: https://core.svn.wordpress.org/trunk@24236 1a063a9b-81f0-0310-95a4-ce76da25c4cd