Movie preview - use frame after 5 seconds
Working on a chunked video can result in a broken frame index the option -vsync 1 can heal that
we better use 5MB (1024 * 1024 * 5 = 5242880) instead of 1MB.
in some cases 1MB was no enough to generate thumbnail
Merge pull request #5882 from owncloud/preview-movie-5s

Movie preview - use frame after 5 seconds