11837.mkv.mp4 [2027]

Since "11837" is often used as a database ID, this file likely originates from:

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. 11837.mkv.mp4

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." Since "11837" is often used as a database