MP4 preview thumbnail not showing up in explorer.

I need a freeware program to cut or trim video's. It doesn't have to be very sophisticated.
But a preview of video and audio tracks is vital.

Windows Mediaplayer 12 doesn't dislay a MP4 video thumbnail with a picture of the actual video.
I only see a standard (one note) thumbnail in explorer.
WMP can play MP4 files.
However, it often fails to play the music or sound added to the video. (wrong file format of plugin?)

There are plenty of video editors but also a bunch of extensions and formats.
I usually take AVI format.

If however I'd like to copy those to my Galaxy, windows recommends a conversion or otherwise it won't work or display on my phone?

All I want is to cut some youtube instruction video to the most vital part and save it as tutorials.

Downloaded Openshot, Shotcut and Avidemux. All have it's problems and limitations where it's a lot of options and fileformats or the lack of opening or transporting other file-formats.
It's possible that the file association of WMP has been altered by another program.
But selecting standard programs and selecting all possibly related extensions to WMP didn't solve the problem either.

It's a known problem but can't find a solution.

Hello Bongo
Thanks for that tip.
I got a part solved with Icaros a program that is capable to restore or associate a bunch of file-extensions. At least the most wanted thumbnails show up now in explorer.
Icaros 3.0.3 / 3.1.0 Beta 2 Download - VideoHelp
Gives you the opportunity to select failing video or audio thumbnails. That is an image of the actual file as preview.
I'm fiddling with the above mentioned programs.
The problem is to find the most common combination of audio and video. As mentioned before, some files won't play either the audio or the video part as the combination isn't right. Or WMP won't and Mediaplayer does, or WinAmp etc.
The compatibility is often curious. I share a small part of a tutorial as demonstration so it must be the most commonly universal file-type.