Invalid image (test.png with 0 Byte) breaks content view

[juergen-venne]

Thumbnails of broken or invalid images are not shown and a ugly missing image is show.
The old codebase handled this as follow:
For every image the Icon of the file type was show to the user in first step. Then the Thumbnail was loaded via Ajax, and on success the Icon was resized to the corner of the image and the thumbnail was shown.
This is maybe also out of scope of this sprint, but we should discuss how to handle that in a user friendly way.