Cannot use Afform to create a linked activity
As per extensions/afform#21 you cannot use Afform to create an activity when the form is submitted because the source_contact_id field is required by APIv4 when doing activity.create / activity.save and there is no way to pass through a param in the Afform form.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information