How do we make attachments mandatory for Internal Supplier Registration?

Summary:

The attachment field (being an object) is unable to get converted into a required field via form Personalization/Sandbox. We need to make the field mandatory when the Business relationship selected in the registration form is 'Spend Authorized'?

Also, if there is no way of making the seeded field as required based on a criteria, how else can we achieve solution for the scenario discussed above?

Content (required):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):