Video files often come with separate subtitle files, especially for content intended for a global audience. The NSFS-324-engsub file you've mentioned seems to follow a common naming convention where:
If you can provide more context — for example: NSFS-324-engsub Convert02-00-52 Min
Intrigued, John and Alex decided to investigate further. They opened the file, which turned out to be a video. To their surprise, it was a documentary about environmental conservation, narrated in English but seemingly produced in another language originally, given the "engsub" in the filename. Video files often come with separate subtitle files,