Description of problem:
From https://github.com/zanata/zanata-server/pull/400
If a user drags a whole folder to the upload drag-and-drop panel, the proceeding upload can have mixed results, including:
- massive performance degradation while traversing the folder
- 

Version-Release number of selected component (if applicable):


How reproducible:
Easy always

Steps to Reproduce:
1. Sign in as a maintainer, go to project
2. Click Settings, Documents, then the [+] button
3. Find a folder via your system file browser that has many (a few hundred+) files of various types
4. Drag that folder to the drop panel in Zanata

Actual results:
Long wait, no feedback

Expected results:
There should be either an instant rejection of the folder (files only) or a constant feedback loop to the user, with the ability to cancel the action.

Additional info:
This may affect things such as paths (duplicate filenames in other dirs), file sizes etc