Change the "No file format specific differences found inside, yet data...

Change the "No file format specific differences found inside, yet data differs" message to be clearer that diffoscope "knows" about this file format yet could not be helpful in this case.

diffoscope/comparators/utils/file.py

@@ -368,8 +368,10 @@ class File(object, metaclass=abc.ABCMeta):
                     if difference is None:
                         return None
                     difference.add_comment(
-                        "No file format specific differences found inside, "
-                        "yet data differs ({})".format(self.magic_file_type),
+                        "Format-specific differences are supported for this "
+                        "file format but none were detected ({})".format(
+                            self.magic_file_type,
+                        ),
                     )
             except subprocess.CalledProcessError as e:
                 difference = self.compare_bytes(other, source=source)