Edited Issue: ReadAsMultipartAsync returns result before all files have been written [282]

We trying to perform validation of the files which have been processed by ReadAsMultipartAsync, but sometimes on the attempts to open file for reading IOException is throwing, like this:

The process cannot access the file 'C:\\Windows\\TEMP\\BodyPart_abd1efe9-b1d5-4956-b79b-0519837e9377' because it is being used by another process.","StackTrace":" at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, String msgPath, Boolean bFromProxy)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)
at System.IO.File.OpenRead(String path)

at ...

at System.ComponentModel.DataAnnotations.ValidationAttribute.GetValidationResult(Object value, ValidationContext validationContext)
at System.Web.Http.Validation.Validators.DataAnnotationsModelValidator.Validate(ModelMetadata metadata, Object container)
at System.Web.Http.Validation.DefaultBodyModelValidator.ShallowValidate(ModelMetadata metadata, ValidationContext validationContext, Object container)
at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateProperties(ModelMetadata metadata, ValidationContext validationContext)
at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateNodeAndChildren(ModelMetadata metadata, ValidationContext validationContext, Object container)
at System.Web.Http.ModelBinding.FormatterParameterBinding.<>c__DisplayClass1.<ExecuteBindingAsync>b__0(Object model)
at System.Threading.Tasks.TaskHelpersExtensions.<>c__DisplayClass40.<ToAsyncVoidTask>b__3f()
at System.Threading.Tasks.TaskHelpers.RunSynchronously[TResult](Func`1 func, CancellationToken cancellationToken)

Example code:

var provider = new MultipartFormDataStreamProvider(_uploadPath);
return content.ReadAsMultipartAsync(provider)
.ContinueWith(multiPartTask =>
{
if (multiPartTask.IsCanceled || multiPartTask.IsFaulted)
{
Trace.TraceError("Failed multipart message reading: {0}", multiPartTask.Exception);
return null;
}

// ...
// use files in the task Result
// using (var fileStream = File.OpenRead(fileName)) {}

return null;
});

When processing time is delayed by something, eg Thread.Sleep, count of the io exceptions decreased, but it's not workaround.
How can we get access to the files, when it's actually written?