Making Absence Certification Mandatory or Attachment Mandatory with custom message displayed

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Hi

We have a requirement to capture the medical documents when employee submits a absence type medical and approval should go only to HRs and not to managers.

Now to achieve this should I be using certifications ? or Make attachment mandatory under Primary Absence details. ?

Which is the more recommended way and what would be the pros and cons of using either.

How do I put a custom error message when employee submits the leave for not to accept if there is no attachment.

Also how do I setup approval flow only to HRs as I am seeing when the create absence type of Illness an Injury by default Approvers is taken as 'Manager of employee's primary assignment.