At the service, you can attach files to a post by selecting them on your device, or pull them to a post by clicking on a link. In order to pull a file to the post by the link, just place the link to the file in the window for the text of the post. You will see how the file will be attached and available for editing. If you attach a video, it will be available for editing only after saving the post.
After attaching the file, you can erase the link address from the text field and place the text for your post there.