A post-function is a bit late in the cycle to do this - post-functions run when a transition is committed to.
You need a validator to check that an attachment has been added. A validator simply returns a "pass or faile" result to JIRA, with pass meaning "yes, go run the post-functions and update all the data" and fail meaning "no, return the user back to the screen they were on and tell them what they've done wrong"
There are several add-ons that provide a validator on attachments in the marketplace. Most of them just check that there is at least X attachments, and if you want to do something more clever (like check the type, or have a look at the content), you'll need something like the Script Runner or your own code.
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