This is actually in the project roadmap. Supporting task attachments is already supported in the engine, but not in the tooling. My guess will be that this will be tackled for 6.1.
There are some alternatives, like for example creating another screen to upload files and then linking them using a task variable.
If you are interested in helping us out to get this done, we can talk offline and coordinate the work.
Please provide more details about this (creating another screen to upload files and then linking them using a task variable.) so that I can give a try.