[HttpPost]
public Task ImgUpload()
{
    // 检查是否是 multipart/form-data
    if (!Request.Content.IsMimeMultipartContent("form-data"))
        throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
    //文件保存目录路径
    string SaveTempPath = "~/SayPlaces/" + "/SayPic/SayPicTemp/";
    String dirTempPath = HttpContext.Current.Server.MapPath(SaveTempPath);
    // 设置上传目录
    var provider = new MultipartFormDataStreamProvider(dirTempPath);
    //var queryp = Request.GetQueryNameValuePairs();//获得查询字符串的键值集合
    var task = Request.Content.ReadAsMultipartAsync(provider).
        ContinueWith(o =>
        {
            Hashtable hash = new Hashtable();
            hash["error"] = 1;
            hash["errmsg"] = "上传出错";
            var file = provider.FileData[0];//provider.FormData
            string orfilename = file.Headers.ContentDisposition.FileName.TrimStart('"').TrimEnd('"');
            FileInfo fileinfo = new FileInfo(file.LocalFileName);                   
            //最大文件大小
            int maxSize = 10000000;
            if (fileinfo.Length <= 0)
            {
                hash["error"] = 1;
                hash["errmsg"] = "请选择上传文件。";
            }
            else if (fileinfo.Length > maxSize)
            {
                hash["error"] = 1;
                hash["errmsg"] = "上传文件大小超过限制。";
            }
            else
            {
                string fileExt = orfilename.Substring(orfilename.LastIndexOf('.'));
                //定义允许上传的文件扩展名
                String fileTypes = "gif,jpg,jpeg,png,bmp";
                if (String.IsNullOrEmpty(fileExt) || Array.IndexOf(fileTypes.Split(','), fileExt.Substring(1).ToLower()) == -1)
                {
                    hash["error"] = 1;
                    hash["errmsg"] = "上传文件扩展名是不允许的扩展名。";
                }
                else
                {
                    String ymd = DateTime.Now.ToString("yyyyMMdd", System.Globalization.DateTimeFormatInfo.InvariantInfo);
                    String newFileName = DateTime.Now.ToString("yyyyMMddHHmmss_ffff", System.Globalization.DateTimeFormatInfo.InvariantInfo);
                    fileinfo.CopyTo(Path.Combine(dirTempPath, newFileName + fileExt), true);
                    fileinfo.Delete();
                    hash["error"] = 0;
                    hash["errmsg"] = "上传成功";
                }
            }
            return hash;
        });
    return task;
}

网站名称：ASP.netWebAPI上传图片
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