<form action="xx.php" method="post" enctype="multipart/form-data">
  <input type="text" name="uname" class="uname" /><br />
  <input type="text" name="email" class="email" /><br />
  <input type="file" name="file" class="file" /><br />
  <input type="submit" name="submit" value="提交"/>
</form> 

using (var client = new HttpClient())
using (var content = new MultipartFormDataContent())
{
  client.BaseAddress = new Uri("http://localhost/WapAPIExp/");
  var fileContent1 = new ByteArrayContent(File.ReadAllBytes(@"D:\xx.jpg"));
  fileContent1.Headers.ContentDisposition = new ContentDispositionHeaderValue("file")
  {
    FileName = "xx.jpg"
  };
  var dataContent = new ByteArrayContent(Encoding.UTF8.GetBytes("1"));
  dataContent.Headers.ContentDisposition = new ContentDispositionHeaderValue("form")
  {
    Name = "type"
  };
  content.Add(fileContent1);
  content.Add(dataContent);
  var result = client.PostAsync("api/Upload", content).Result;
}