WAF: Is it possible to disable adding a content-type by reverse proxy?

Hi All,

Issue:

We have an issue with publishing Team Foundation Server through the Sophos UTM.  During code checkin from a Visual Studio Client 2015 users get an error "unsupported content-type application/x-ashx" The second time they push the checkin button everything goes well.
Older clients seem to just ignore the content-type header and do not show this error.

I have tracked it down to the Sophos UTM reverse proxy adding the content-type: application/x-ashx header. A direct connection to the Team Foundation Server does not return the content-type header and everything is well for the Visual Studio 2015 Client. A test setup with the Microsoft ARR reverse proxy also does not add / return the content-type header. Obviously Microsoft is not following RFC's for HTTP traffic, not on the client and not on the server. But this error is very annoying and user think they checked in their code but forget about the fact that they have to push the button twice because of the error. So code is not being checked in which causes al kinds of problems.

Now the question:

Is it possible to disable the adding of a content-type header for a specific WAF rule?