Update mediafile to 0.13.0 (#5494)

Fixes #5420.

Author: snejus

beets/test/helper.py

@@ -900,7 +900,7 @@ def run(self, *args, **kwargs):
         super().run(*args, **kwargs)
 
     IMAGEHEADER = {
-        "image/jpeg": b"\x00" * 6 + b"JFIF",
+        "image/jpeg": b"\xff\xd8\xff" + b"\x00" * 3 + b"JFIF",
         "image/png": b"\211PNG\r\n\032\n",
     }