Hi Lee,
So we don't have that supported in standard at the moment so it would have to be customised.
You could start by taking a look at how we create attachments in general (e.g. when creating email from Invoice we create pdf attachment). Then the next step would be probably to create a similar function in HAL, and then when you submit the HTML form with uploaded file, we would call this function in HAL.
You could put through a wish for this as well as I am sure this would be good to have in the standard product version.
I hope this is useful and makes sense.