Minor improvements to file upload.

Updated file upload so that the icon of the new upload item reflects if it is a file or notebook, added the ability to use the "Enter" key in the filename text box to cause the upload to happen, and make it so that the list of files refreshes immediately upon successful upload.
Added ESC keep to upload textbox to cancel.
Uploading a file with a name that already exists asks the user if they want to overwrite.

This is not perfect (it doesn't check against the real filesystem but the current list in the browser which may be stale) but it is better than nothing.
Fixed many edge cases in file uploads.

When there is an error reading a file, a message is shown (I could reproduce this easily by dragging a folder onto the list), added a missing "return false;" after an illegal notebook was detected, and we prevent no-name files and files starting with dots (which will not be visible in dashboard and automatically overwritten).
File list refreshes no longer move the upload filename boxes.

The new file list is always placed after the upload filename boxes. I also removed the autorefresh guards against refreshing while having an upload box since this is no longer a problem.
Merge pull request #6306 from coderforlife/master

Minor improvements to file upload.