Image editing: Can't select a file as a thumbnail for an image (regression)

Selecting a file for a thumbnail doesn't upload/save/use the file.

To reproduce:

• Log in to CMS, and click "To the documents"
• Click the "Images" section
• Browse to an image and edit it
• Click "Select file" under one of the thumbnails
• Select an image file to upload

Expected result: The file is uploaded and used as thumbnail
Actual result: The file name is listed under the thumbnail, but is not used. Clicking the red 'delete' cross doesn't do anything. Saving, Closing and editing it again shows that it is not saved or used.