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."
Open it with VLC Media Player . VLC ignores the extension and looks at the actual data bits to play the file.
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.
If you are having trouble playing or using "11837.mkv.mp4," try these steps:
While "11837" is a generic numeric identifier, the double extension tells a specific story about the file's technical history and how it interacts with media players. The Mystery of the Double Extension
To understand this file, you have to look at the two "containers" involved: MKV (Matroska) MP4 (MPEG-4 Part 14) Open-source, flexible, "the Swiss Army knife." Universal, streamlined, "the Industry Standard." Capabilities Supports unlimited tracks (multiple audios/subtitles). Better hardware acceleration and web streaming. Compatibility Great for PCs (VLC); poor for older Smart TVs/Apple. Works on almost every device (phones, consoles, TVs). Common Use Cases for "11837"
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?