DotNetOpenAuth does not verify the value of Content-Type header. As per RFC 6749, it should be set to "application/x-www-form-urlencoded" #376

As per "The OAuth 2.0 Authorization Framework" document RFC 6749,

The client makes a request to the token endpoint by adding the following parameters using the "application/x-www-form-urlencoded" format per Appendix B with a character encoding of UTF-8 in the HTTP request entity-body.

I can send the parameters (such as grant_type, refresh_token, etc) using form-data with Content-Type set as "multipart/form-data; boundary=----WebKitFormBoundaryIog37oV1UGpBc215" instead of "application/x-www-form-urlencoded" and get an access token back successfully.

Also, WebServerClient.ProcessUserAuthorization method does not set the content type to "application/x-www-form-urlencoded" as well.