There isn't a way to restrict the attachments people can upload, by blacklisting a file type. I've found this request on our issue tracker, closed as "won't fix":
There are some suggestions that may be useful in this post:
I hope this helps!
The "solved" flag is for "there's an answer that tells us nothing else needs doing". Mostly that means that the original question has an answer, but it can also mean "closed because there is no answer", or, as in this case, "closed because there's an issue which tells you the state of the request"
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