Originally Posted by Krystian Galaj
Many programs recognize file type based on content. Most file formats have fixed headers, easy to recognize. There could even be a GPL library for that lying around.
I am aware of this, yes. But it's such a trivial thing that brings so little to the end user and requires a substantial amount of effort to develop to a point where its mostly foolproof. There's a huge amount of work to be done on Sigil and since I can devote much less time to it than I'd like, I'd rather spend it on something more useful.