You can only upload documents from the webpage interface.
Website
If you are already working on a form, and have to attach documents on the fly, here's how you can do it:
Only users with corresponding inspection permission to edit and update the form will be able to attach documents. If users have "Read Only" permission for inspection, he can only see the documents but he cannot upload or edit documents.
The documents attached is separated from project document repository. They are only attached to the inspection itself.
Once the form is closed , users are unable to upload documents or to edit the document attached.
Learn more about how to mark document as "Not in Use" here.