How can we pop-up Alert Message if their is No Attachment during Form Submit?

How can we pop-up Alert Message if their is No Attachment during Form Submit?

Dear All,

We have made the Attachment field Mandatory for a Service Template but we also need to pop-up the alert message for the user if he/she doesn't attached the required attachments during the Form Submit.

Please let us know how can we achieve this scenario?

Regards,
Wajahath Farooq 

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