The most curious part of this filename is the double extension: .mkv.mp4 .
This usually happens due to a naming error during a file conversion. The file is almost certainly an MP4 (since that is the final extension), but the creator likely forgot to remove the ".mkv" tag after converting it from a Matroska format. 4261 - BOLT - 2008 - BRRip - 1280x720.mkv.mp4
A versatile container that can hold multiple audio tracks (different languages) and subtitle files. The most curious part of this filename is
A BDRip is encoded directly from a Blu-ray disc. A BRRip is a "re-encode"—meaning it was created from an already existing BDRip or a high-quality digital source. 4261 - BOLT - 2008 - BRRip - 1280x720.mkv.mp4
This indicates the "High Definition" (HD) standard of the video.