Update phorum.rb migration script (PR #14725)

Add attachment/file/upload handling to bring them in from phorum to discourse


CLA assistant check
Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
You have signed the CLA already but the status is still pending? Let us recheck it.

This pull request has been mentioned on Discourse Meta. There might be relevant details there:

Thanks for doing this!