Many features (e.g. Atrium Blog and Atrium Documents) allow you to attach files to content. In that case, you have access to the file attachments field.
This field has an file upload box. Click Browse to select the file you want to upload. The file will be automatically attached to the form. Make sure the file adheres to the file size and file type constraints. A file which is too big or of an unauthorized type will trigger an error message. You can attach multiple files to a book page. You can change the order in which they are listed by clicking and holding the cross icon and dragging the field to a new place in the list. Files you have uploaded will only be saved and attached when you save the entire form. If you close the form without saving, the uploaded files will not be kept.