API Method to accept different file types (for upload)

I've been using Web Api in it's most basic form for a while and got to step out of the scope of how I used to do things.

Need to create a method that will take in a file that need to be processed.

This file might be an Xml file, flat file, excel document, etc (for now it's only an XML file...got to get that working)

Since I must do a "test" on the data to decide if this is an xml file, or this or that, etc I'm not sure what my signature should look like.

I got this now (hoping code describe what I'm trying to get sorted out):  

        public HttpResponseMessage UploadDocument(HttpRequestMessage request)
        {
            // what will be returned eventually
            var result = new HttpResponseMessage();     
            // use factory to decide which "handler" will process the posted document (xml, flat, excel, etc)
            IImportHandler handler = ImportFactory.GetHandler(request.Content);

            bool valid = handler.ValidateData(request.Content);

So in a nutshell, I'm asking what should the input be and how to determine if it's an xml document or not?