上传使用MultipartFormDataStreamProvider的图像时出错

Question

我实现了上传功能：

MimeMultipart.cs

public class MimeMultipart : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (!actionContext.Request.Content.IsMimeMultipartContent())
            {
                throw new HttpResponseException(
                    new HttpResponseMessage(
                        HttpStatusCode.UnsupportedMediaType)
                );
            }
        }

        public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
        {

        }
    }

FileUploadResult.cs

public class FileUploadResult
    {
        public string LocalFilePath { get; set; }
        public string FileName { get; set; }
        public long FileLength { get; set; }
    }

UploadMultipartFormProvider.cs

public class UploadMultipartFormProvider : MultipartFormDataStreamProvider
    {
        public UploadMultipartFormProvider(string rootPath) : base(rootPath) { }

        public override string GetLocalFileName(HttpContentHeaders headers)
        {
            if (headers?.ContentDisposition != null)
            {
                return headers
                    .ContentDisposition
                    .FileName.TrimEnd('"').TrimStart('"');
            }

            return base.GetLocalFileName(headers);
        }
    }

UploadController.cs

[MimeMultipart]
[Route("images/upload")]
public HttpResponseMessage Post(HttpRequestMessage request, int companyId)
{
    return CreateHttpResponse(request, () =>
    {
        HttpResponseMessage response = null;

        var companyOld = _companiesRepository.GetSingle(companyId);
        if (companyOld == null)
            response = request.CreateErrorResponse(HttpStatusCode.NotFound, "Invalid company.");
        else
        {
            var uploadPath = HttpContext.Current.Server.MapPath("~/Content/upload/companies/images");

            var multipartFormDataStreamProvider = new UploadMultipartFormProvider(uploadPath);

            // Read the MIME multipart asynchronously 
            Request.Content.ReadAsMultipartAsync(multipartFormDataStreamProvider);

            string localFileName = multipartFormDataStreamProvider
                .FileData.Select(multiPartData => multiPartData.LocalFileName).FirstOrDefault();

            // Create response
            if (localFileName != null)
            {
                var fileUploadResult = new FileUploadResult
                {
                    LocalFilePath = localFileName,

                    FileName = Path.GetFileName(localFileName),

                    FileLength = new FileInfo(localFileName).Length
                };

                // update database
                companyOld.Logo = fileUploadResult.FileName;
                _companiesRepository.Edit(companyOld);
                UnitOfWork.Commit();

                response = request.CreateResponse(HttpStatusCode.OK, fileUploadResult);
            }
        }

        return response;
    });
}

Uploaded Image：图片大小101KB

Image after upload：图片大小31.8KB

错误：图像尺寸自动掉落。