if(Request.Content.IsMimeMultipartContent())
{             
   var streamProvider =newMultipartMemoryStreamProvider();
   var task =Request.Content.ReadAsMultipartAsync(streamProvider).ContinueWith(t =>//上传多个文件
   {
     foreach(var item in streamProvider.Contents)
     {
       //do something
     }
});

public class UploadFileController : ApiController
    {
        public async Task<string> Post()
        {
            // 检查是否是 multipart/form-data
            if (!Request.Content.IsMimeMultipartContent("form-data"))
                throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);

            // 设置上传目录
            var provider = new MultipartFormDataStreamProvider(@"c:\\temp");
        
            // 接收数据，并保存文件
            var bodyparts = await Request.Content.ReadAsMultipartAsync(provider);

            string result = "";
            // 获取表单数据
            result += "formData txtName: " + bodyparts.FormData["txtName"];
            result += "<br />";

            // 获取文件数据
            result += "fileData headers: " + bodyparts.FileData[0].Headers; // 上传文件相关的头信息
            result += "<br />";
            result += "fileData localFileName: " + bodyparts.FileData[0].LocalFileName; // 文件在服务端的保存地址，需要的话自行 rename 或 move

            return result;
        }
    }