You can add a file to your Brain from Safari or other location by sharing it with TheBrain. This will add it to TheBrain's inbox where it can be added to a Thought as an attachment.
To add a file from Safari, open the document in Safari then tap anywhere on the document to bring up the Open sub-menu. Tap Open In...
Tap Open In TheBrain.
You will see a brief notification that your item was added.
Tap the attachment icon in the toolbar section of TheBrain.
Tap Open Inbox.
You will see the item you added listed in TheBrain Inbox. You can tap Close Inbox if you decide you do not want to add the item. Tap the item you wish to add as an attachment and you will be presented with a menu of options.
Attach will attach the item to your Thought. Preview will open the item and allow you to view it. Close will close the menu.
Tap Attach and you your item will be attached to your Thought.