[BUG] Blank Video Thumbnail After Converting Video.

[Selur]

Did some testing on my Win10 system and there except for H.264 inside mp4 thumbnails are shown. (tested MPEG-4 H.265, Mpeg-4 H.264 and MPEG-4 ASP)
Tried both mp4box&ffmpeg as muxer, x264 inside mp4 gets no thumbnails here.

Cu Selur