Unable to upload a new attachment using the "All pages" tab in the WYSIWYG editor

Steps to reproduce:

• Login as a user or admin
• Navigate to a page and edit it using the WYSIWYG editor
• Click on Link -> Attached File...
• Click on the "All pages" tab
• Navigate to any page, click on the + sign to its left, then click on the + sign corresponding to the "Attachments" field
• Click on "Upload file..."
• Click on "Select"

After clicking "Select" you will see a continuous loading screen.

Tested on IE8, IE9, Chrome 23 and Firefox 16.0.2.