Preview images of media files with .webm and .ogg formats are not loaded

[jasson99]

Steps to reproduce

1. Upload a file sampleOgg.ogg or sampleWebm.webm
2. The preview image of the uploaded file is not visible
   

Expected behaviour

The preview image of media files with .ogg and .webm formats should be visible

Actual behaviour

The preview images are not visible

[fschade]

Need to be implemented in thumbnails service. For videos it's hard to decide which frame should be used for preview. I leave this open for discussion and collecting ideas.

cc: @C0rby

[C0rby]

In oc10 we display a generic video icon for video files.
I would propose to do the same for oCIS since thumbnailing for videos is a bit more complex.

[pascalwengerter]

Closing this in favor of #4667