We are currently using a deprecated message flag to avoid accumulating the chunks that are downloaded. We should use the new API. There is also a commented out FIXME line for using this feature when uploading files, which can now be enabled.
Created attachment 28360 [details] replaces the deprecated flag
Created attachment 28361 [details] disable accumulating for file uploads
Comment on attachment 28360 [details] replaces the deprecated flag yes!
Both landed, as r41482 and r41483.