Fix bug where no content-type is provided in a message part

dmarcreceiver/commands.py

@@ -122,6 +122,10 @@ def receive_report(args):
     for part in msg.walk():
         # check for zip file
         content_type = part['content-type']
+
+        if content_type is None:
+            continue
+
         if content_type.find(';') != -1:
             content_type = content_type.split(';',1)[0]