Throw exception when Content-Type is not specified

I have a Web API project that returns some product data.

If no Accept header is specified it returns XML as default, done like so in my WebApiConfig:

config.Formatters.Clear();
config.Formatters.Add(new XmlMediaTypeFormatter());
config.Formatters.Add(new JsonMediaTypeFormatter());
config.Formatters.Add(new FormUrlEncodedMediaTypeFormatter());

So default is XML, the first formatter, but the API still supports JSON if the request asks for it.

In my ControllerHelper, I added a 415 Format not supported response:

catch (FormatException) 
{
    var resp = new HttpResponseMessage(HttpStatusCode.UnsupportedMediaType) 
    {
        Content = new StringContent(string.Format(HttpStatusCode.UnsupportedMediaType.ToString())),
    };
    throw new HttpResponseException(resp);
}

And I would like to throw that response if no Accept header is specified and therefore require it to be set, either to application/xml, text/xml or application/json.

For example, if I test to set accept in Advanced Rest Client to application/foo I want to throw an exception.

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How to do this? Thanks in advance!