Add attachments in a form is now possible. The first option is by using the new feature from article How to create child items from a parent form. The second option is by adding the attachment column in the form. Benefit of the first option is to add attachments in a searchable library. Benefit of the second option is to be able to attach a document or image when registering an event.
In the list settings, advanced settings, make sure attachments are enabled. Open the BPA form as a power user and open the form settings. Click + to add the attachment column.
When registering a new item, click attachment and select one or many attachments. Attachments are visible when opening the form. Also, attachments can be viewed and opened from the master detail component.
As a conclusion, users can register events including attachments and photos in a single step.