If you as a Premium user share a project with a free user, then the only additional option they will have is to use comments in tasks. Other than that, they will not gain any Premium features.
In case of file attachments, only you as a Premium user will be able to upload a file to a comment. The free user will see the file and have access to it, but they can't upload files through their accounts.
How about tags and filters ?
Thanks in advance
Tags and filters are already available for non-premium user. Did you have a specific feature in mind?
We have premium only feature relating to tags and filters but as mentioned by David, free users wouldn't gain any Premium features aside from the exceptions listed above.