public class UploadController : Controller
    {
        private IHostingEnvironment _hostingEnv;
        public UploadController(IHostingEnvironment hostingEnv)
        {
            _hostingEnv = hostingEnv;
        }
        [HttpPost]
        public IActionResult Index()
        {
            var file = Request.Form.Files;           
            if (file.Sum(f => f.Length) > 0)
            {
                foreach (var pic in file)
                {
                    var picname = ContentDispositionHeaderValue
                                .Parse(pic.ContentDisposition)
                                .FileName
                                .Trim('"');
                    var exname = picname.Substring(picname.LastIndexOf("."));
                    var picfullname = DateTime.Now.ToString() + exname;
                    picname = _hostingEnv.WebRootPath + $@"\UploadImg\{picfullname}";

                    using (FileStream fs = System.IO.File.Create(picname))
                    {
                        pic.CopyTo(fs);
                        fs.Flush();
                    }
                }
            }
           
            return Content("ok");
        }
    }

 public class UploadController : Controller
    {   
        [HttpPost]
        public ActionResult Index(HttpPostedFileBase picture)
        {
            if (picture.ContentLength > 0)
            {             
                string filePath = Path.Combine(HttpContext.Server.MapPath("../UploadImg"),
                   Path.GetFileName(picture.FileName));
                picture.SaveAs(filePath);                                                 
            }
            return Content("ok");
        }
    }

<form method="post" enctype = "multipart/form-data">
 <div class="form-group">
<label class="control-label col-md-2">预览图</label>
<div class="col-md-4">
  <div class="fileupload fileupload-new" data-provides="fileupload">
    <div class="fileupload-new img-thumbnail" style="width: 200px; height: 150px;">
       <img src="~/Images/AAAAAA&amp;text=no+image.png" />
               </div>
      <div class="fileupload-preview fileupload-exists img-thumbnail" style="width: 200px; max-height: 150px"></div>
       <div>
     <span class="btn btn-default btn-file"><span class="fileupload-new">选择图片</span><span class="fileupload-exists">更换</span><input type="file" name="pic" id="picture"></span><a class="btn btn-default fileupload-exists" data-dismiss="fileupload" href="#">清除</a>
      </div>
    </div>
      </div>
 </div>
</form>