Upload tasks from NSData are not supported in background sessions

[littleben803]

When I use startUploadRequest to upload image data to server, it throw a exception:
"Upload tasks from NSData are not supported in background sessions"

the code is such like that:
//------------------------------------------------------------------------------------------------------------------------------
MKNetworkRequest *request = [self.host requestWithPath:@"MyUrl" params:finalSendParams httpMethod:@"POST"];
[request attachData:data forKey:@"img" mimeType:@"image/jpeg" suggestedFileName:@"img.jpg"];
[self.host startUploadRequest:request];
//------------------------------------------------------------------------------------------------------------------------------

in function -(void) startUploadRequest:(MKNetworkRequest*) request;
use the backgroundSession like that:
request.task = [self.backgroundSession uploadTaskWithRequest:request.request
fromData:request.multipartFormData];

maybe we can only upload files "fromFile" in backgroundSession rather then "fromData".