Previews/Thumbnails for pictures

[PhMemmel]

The repository listing should show previews/thumbnails of image files. Of course it has to be assured that this does not have negative impact on the performance (for example for directories with a lot of pictures).

Priority probably pretty low :)

[individual-it]

This should be an configurable option, so that admins can switch it off. oCIS already allows to get thumbnails for png, jpg, gif, tiff, bmp & txt files see https://owncloud.dev/services/thumbnails/
So with a lot of files the load would be on the ocis server and the traffic will of course increase