Bug 418585

SUMMARY
Our Windows users get really confused about how to get the correct ffmpeg. This needs a step-by-step tutorial (with "how to get a release version instead of a development build" and "how to extract the executable") on Krita Manual, but it would be good to also detect whether someone try to give Krita the .zip file instead of .exe (or just ffmpeg on Linux).

STEPS TO REPRODUCE
1. Download ffmpeg from the https://ffmpeg.zeranoe.com/builds/ site, especially the already suggested one (development build) to see the purpose of Additional Notes
2. Note you downloaded a .zip file
3. Provide the path to the .zip file as a path to ffmpeg in Krita
4. Try to render an animation

OBSERVED RESULT
No warning, failed rendering, users are confused since they downloaded ffmpeg exactly as the Krita Manual says.


EXPECTED RESULT
If the filename ends with ".zip", there should be a warning with a short instruction, maybe something like: "The ffmpeg file you provided the path to has a '.zip' extension. If you encounter issues while rendering, check if the file is an archive, extract all files and point Krita into bin/ffmpeg.exe file inside the folder with files from the archive" or something similar.


ADDITIONAL INFORMATION
Would be awesome to check if the ffmpeg used is a release build or not... I believe most Windows users use the development build now -.- since that's what ffmpeg suggests by default... Maybe not a warning, but some kind of [!] icon next to the ffmpeg path with a tooltip explaining why?


SOFTWARE/OS VERSIONS
Windows: 10
Krita 4.2.8