Model always null on XML POST

Two things:

1. You don’t need quotes "" around the content type and accept header values in Fiddler:

   User-Agent: Fiddler
   Content-Type: application/xml
   Accept: application/xml
   

2. Web API uses the DataContractSerializer by default for xml serialization. So you need to include your type’s namespace in your xml:

   <TestModel 
   xmlns="http://schemas.datacontract.org/2004/07/YourMvcApp.YourNameSpace"> 
       <Output>Sito</Output>
   </TestModel> 
   

   Or you can configure Web API to use XmlSerializer in your WebApiConfig.Register:

   config.Formatters.XmlFormatter.UseXmlSerializer = true;
   

   Then you don’t need the namespace in your XML data:

    <TestModel><Output>Sito</Output></TestModel>