Constraint/Validate if File Attachment Attribute is none

Summary

If file attachment attribute contains no file, Constraint/validate should throw an message

Content

We have a file attachment attribute for storing the Proposal that is generated. We are sending the proposal to customer in mail using the Steps-> Define notification ( Email Template and Attachment are added). Once the manager approves the quote, the quotation can be sent to customer using 'Send to Customer' (modify action) button. Once the action is clicked, Email will be sent to customer along with Proposal Document. But if user forgets to click Generate Proposal before sending it to customer, mail will be sent without Proposal Attachment.