Is FDM supposed to be able to preview MP4 files ?
Whenever I have tried, it says "Failed to open the file. Unknown Media Type".

FDM may display this error message if the file is broken (incomplete or containing garbage), unavailable (blocked by antivirus, moved to another place with external file manager, etc.) or in another format (you may have downloaded small HTML file or some archive instead - rar/zip/gz etc.).
Double click the mp4 file on the download list and try to play it using any external media player. Does it work? If not, right click the file name on the download list and select "Open folder" from the context menu. Does it work?

Well, it seems that FDM preview feature is not fixed yet for Unicode support. You can preview media with filenames in English (or other language using Latin alphabet without accented characters) and language set as default in Windows. You probably cannot preview media if filenames contain some accented or non-Latin characters, f.e. in Russian or Japanese.
For example in my Windows set to Polish language I can preview files with names in English and Polish, but not in Russian.

It isn't fixed in Build 1627 either, There were no non-western characters in the filename either. It was just a bog standard MP4.

What standard? H264, H265, anything else? Or maybe it's ts, m2ts or some MPEG DASH stream with wrong file extension?

malau wrote:Is FDM supposed to be able to preview MP4 files ?
Whenever I have tried, it says "Failed to open the file. Unknown Media Type".

Hello!

Please provide sample URLs of the videos where you encountered the issue.

Regards,
Clive