Upload files in chunks to avoid failing from server timeout.

[chrisbobbe]

See zulip/zulip#9391, where Tim pointed out that clients should upload files in chunks.

I think it's quite plausible that the timeouts are part of #4340, which has #4475 as a partial fix to get some of the inflated JPEG files under some control.

I don't have an implementation plan right now; I'm just filing this so I don't forget about it.