Currently you cannot attach files from the repository to a form, unless you download/export them to your desktop first.

A while ago, one of the VARs posted a pretty neat write-up of a complicated solution they designed as a workaround to this issue... but it is probably best to wait for Laserfiche to actually develop this feature themselves. The solution (link) involved inserting data directly into the Forms database, which is potentially fragile and may break as the product evolves. It's still a very interesting read!