In order to enable adding attachments to the request, follow these steps:
Go to Configuration → Absence types → select the absence type → edit
Go to Attachment tab
Enable attachments and save the changes.
It is not required to add the attachment to the request, but you can add a custom message that will inform your employees that they should attach a file if they want their request to be accepted.
In order to do that, go to the Messages tab and put your message there.
Your employees will see this information before creating the absence request.