Question

Can you can make an info path attachment control mandatory based on a set criteria?

if listbox selected value is High 
    attachment is mandatory 
else 
    not mandatory .

this form will be in sharepoint as a web based form

Answer 1

No you can’t change the build in mandatory support on the fly because it depends on the xsd schema

But you can just make your field not mandatory and then add a data validation which doesn’t allow the field to be blank if listbox is High