Function opened_as 

pub fn opened_as(bytes: &[u8]) -> (&'static str, &'static str)

What content an app is opened with is taken as, from its bytes: its file extension and media type. A picture or a video by its signature, a document by its format, text as text; anything else is opaque bytes. Content handed to an app travels without the name its sender gave it, so the bytes alone say which app’s file it is.