Here you'll find information about the functionality when using forms and services in Altinn.
Attachment forms and/or file attachments can be added in List - forms and attachments, you can find it on the top left side of the form.
When you have completed a form you need to click Check form to get to the next stage of the process. If everything is in order you will be able to send the form, click Proceed to signing -> Sign and submit.
You will find a copy of your submitted form in the archive. This confirms that the form is sent to the agency which will process it.
Once you have signed and submitted a form, you may get a receipt in your inbox.