Has this ever happened to you?

You spend time lovingly crafting a video, even embedding a custom thumbnail for all your viewing pleasure, only for the your file manager to display a random frame from the video instead of its proper thumbnail? Hell, maybe you just downloaded a YouTube video and you're wondering where the thumbnail has gone.

So you turn to StackExchange, and get answers to the wrong question; yes, you have the codecs installed, you have ffmpegthumbnailer installed; it's not that thumbnails aren't being generated, it's that the wrong thumbnails are being generated; or, rather, thumbnails shouldn't be generated because there already is one. You know it's there; you've checked with video player programs, you've looked at the metadata, but your file manager seemingly just refuses to acknowledge it.

The solution is simple!

^((and feels extremely obvious in retrospect, but I hope this guide can help anyone else having this same niche issue))

1) In preferences, set your preview settings to a. Show thumbnails:Yes b. Only for files smaller than:64GB (this is based on the Nemo file manager; other file managers should have the same settings, if slightly different language used).

2) As root, navigate to /usr/share/thumbnailers/ffmpegthumbnailer.thumbnailer; open it in a text editor—you should see something like this:

[Thumbnailer Entry]
TryExec=ffmpegthumbnailer
Exec=ffmpegthumbnailer -i %i -o %o -s %s -f
MimeType=video/jpeg;video/mp4;video/mpeg;video/quicktime;video/x-ms-asf;video/x-ms-wm;video/x-ms-wmv;video/x-ms-asx;video/x-ms-wmx;video/x-ms-wvx;video/x-msvideo;video/x-flv;video/x-matroska;application/x-matroska;application/mxf;video/3gp;video/3gpp;video/dv;video/divx;video/fli;video/flv;video/mp2t;video/mp4v-es;video/msvideo;video/ogg;video/vivo;video/vnd.avi;video/vnd.divx;video/vnd.mpegurl;video/vnd.rn-realvideo;application/vnd.rn-realmedia;video/vnd.vivo;video/webm;video/x-anim;video/x-avi;video/x-flc;video/x-fli;video/x-flic;video/x-m4v;video/x-mpeg;video/x-mpeg2;video/x-nsv;video/x-ogm+ogg;video/x-theora+ogg

3) See that third line, Exec=ffmpegthumbnailer -i %i -o %o -s %s -f? All you have to do is add -m to the end, and save the file:

Exec=ffmpegthumbnailer -i %i -o %o -s %s -f -m

^((This simply) ^(tells ffmpegthumbnailer to check for a pre-existing embedded image)^(, and to prefer the embedded image if it exists))

4) Navigate to ~/.cache, and delete the thumbnails folder.

And you should be good to go!