Cannot link a Zoho document with a bug. Why?

Cannot link a Zoho document with a bug. Why?

We've decided to use bugs rather than tasks to handle all of our team's assignments since they support agile better.
Unfortunately it appears that currently there's no easy way to link a bug with a Zoho document that has already been added to the project documents.  I think it's a serious oversight. 

A bug's attachments tab allows to upload local files but not to attach Zoho documents.  It should do both.