In attachment control you have set the maximum value equals to "1"
but you still can attach more than 1 file to the attachment control.
Composer 11.x and 12.x
Working as per design.
Attachment control is working but will be triggered after save and route of the solution form.
At first glance it looks like the attachment control will allow multiple uploads as if it was not working:
But will then give validation errors when you save and route and will not allow you to go further and give error message as per below: