Can't uplaod attachment >1mb in slow connection

Can't uplaod attachment >1mb in slow connection, I have got "Http error".
I tried same files in https://www.plupload.com/examples/ and it uploaded successfully.
https://www.plupload.com/examples/ uses 1mb chunks.

I fixed this error in my site by reducing chunks to 512kb.
I suggest to add option to adjust upload chunks or make it max value 512kb.