To answer my own question, in case someone else also strikes this, the issue with attachment uploads came down to the "mimetypes.php" in the config directory for roundcube.
I was initially testing with .txt and .conf files as they were small. Turns out if these aren't in the mimetypes list, RC doesn't know what to do with them and so Apache spit an "error 500" at you.
Once I made the relevant modifications to the mimetypes file, for things like .pdf, .jpg, .zip (i.e. the usual things you normally attach to emails) they all work fine.