Hi Pharos,
I'm sorry to hear you're having problems with this. I'm not exactly sure why this isn't working. It's definitely possible that this is an issue with SoapUI. However, the StackOverflow thread that Marcus linked hints towards this also being an issue (in some cases; in others it seems to work perfectly - perhaps that is why postman is working?) with asp.net mvc 3. Is that the version which you are using? If so, have you tried doing this with a web api set up with MVC 4?
We unfortunately currently don't have the environment to reproduce this, so I just want to narrow it down as far as possible before we start the investigation.
Regards,
Arian
SmartBear Sweden
