Function fz_recognize_document_stream_content 

pub unsafe extern "C" fn fz_recognize_document_stream_content(
    ctx: *mut fz_context,
    stream: *mut fz_stream,
    magic: *const c_char,
) -> *const fz_document_handler

Given a magic find a document handler that can handle a
document of this type.

stream: the file stream to sample. May be NULL if the document is
a directory.

magic: Can be a filename extension (including initial period) or
a mimetype.