Many batch-processing tools (like Handbrake or FFmpeg) create a new file based on the original name. If the original was 11837.mkv , the output becomes 11837.mkv.mp4 . MKV vs. MP4: What’s Inside?
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
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. MP4: What’s Inside
Files shared on messaging platforms are often assigned long numeric strings to avoid name conflicts on the server. How to Handle This File Note: Renaming an extension does not actually convert