Use request.mimetype instead of request.content_type

request.content_type has encoding and other information associated
with it. If we send in multipart/form-data, the content_type attribute
is not just the mime type which is what we are interested in here, but
is a string of the form multipart/form-data;...

Author: amitsaha

graphql_flask/graphqlview.py

@@ -172,4 +172,6 @@ def format_error(error):
 
     @staticmethod
     def get_content_type(request):
-        return request.content_type
+        # We use mimetype here since we don't need the other
+        # information provided by content_type
+        return request.mimetype