11837.mkv.mp4 -

A user might have tried to change the file format by simply typing .mp4 at the end of an .mkv file. Note: Renaming an extension does not actually convert the data inside; it only changes how your computer tries to open it.

Open it with VLC Media Player . VLC ignores the extension and looks at the actual data bits to play the file. 11837.mkv.mp4

Since "11837" is often used as a database ID, this file likely originates from: A user might have tried to change the

Use a tool like MediaInfo . It will tell you if the file is truly an MP4 or if it's just an MKV wearing an MP4 "mask." 11837.mkv.mp4