c# - HttpContent PostAsync Different Signatures

Question

I am calling an external POST API via HttpClient.PostAsync object however, the API has 3 signatures

    [HttpPost]
    public virtual ActionResult Up(string a, HttpPostedFileBase b, string c)
    {
    }

Given the parameters, I cannot create a StringContent (derived from HttpContent) because the one of the parameters is not string to be serialized.

How will I be able to pass an HttpContent object to PostAsync calling the Up method given the signatures?

 HttpClient client = new HttpClient();
//StringContent queryString = new StringContent(data);

HttpResponseMessage response = await client.PostAsync("/api/up", //*How do I Insert an HttpContent object here );

HttpPostedFileBase Object in API parameter is not string btw.