Have an expanded acronym of "Portable Network Graphics"
From byte 0, all PNG files have “\x89PNG\r\n\x1A\n” in the file content
Match a file glob pattern of “*.png” (case insensitive)
There are hundreds of entries like this in the XML with formal categorization :)
It sounds like you’re trying to approach a solved problem, though. Why are you building a file list, and what are you going to use it for? What are you ultimately trying to do?