It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
The chat quoting mechanism will need to be able to generate
markdown for all kinds of uploads. The UploadMarkdown class
was missing generation for video and audio uploads. This
commit adds that in, and also expands the server-side regex
recognition of FileHelper types to match those in uploads.js,
and adds a spec for UploadMarkdown
