Forms that submit attachments should have a enctype attribute on the form element set to "multipart/form-data" but the form engine will automatically add it if you forget.
Attachments can be added to forms by simply setting the type attribute to "file." All the normal visibility settings apply. When approvers are on the Active Form Queue screen they will have a drop down menu displaying all attachments for quick access. They will also be able to download attachments from the form itself. When editing a saved for section attachments can be downloaded or deleted. Delete an attachement first to upload a replacement.
If you're using Bootstrap styles with the Forms jQUery Plugin you can replace the default file upload field with a field that matches Bootstrap styles by adding the "fancyupload" class to the input field.