Expose thumbnail file to extensions

This is needed by Media Viewer in order to display a placeholder
image as soon as possible

See https://gerrit.wikimedia.org/r/#/c/121613/

Change-Id: I3dfa80b02073984dc2d3d7784cb3744b7e4e2cc8
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/293
Merge "Expose thumbnail file to extensions"