comparators.utils.file: Include magic file type when we know the file format...

comparators.utils.file: Include magic file type when we know the file format but can't find file-specific details. (Closes: #850850)

Signed-off-by: Chris Lamb <lamby@debian.org>

diffoscope/comparators/utils/file.py

@@ -213,7 +213,7 @@ class File(object, metaclass=abc.ABCMeta):
                         return None
                     difference.add_comment(
                         "No file format specific differences found inside, "
-                        "yet data differs",
+                        "yet data differs ({})".format(self.magic_file_type),
                     )
             except subprocess.CalledProcessError as e:
                 difference = self.compare_bytes(other, source=source)