make AFHTTPRequestOperationManager can eat text/plain (or other content-type)
rmb re-init serializer for it…
the default one like JSON very much … but i m not doing JSON …

AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
manager.responseSerializer = [AFHTTPResponseSerializer serializer];
manager.responseSerializer.acceptableContentTypes = [NSSet setWithObject:@”text/plain”];

rmb add “file://” when using multipart form file
NSString *file = [[NSString alloc] initWithFormat: @”file://{file_address}];