I see that you have found Service Desk is storing these attachments into a .dat file. Perhaps these have a filename such as winmail.dat
My colleague @Angélica Luz created a helpful article about this problem in https://community.atlassian.com/t5/Jira-Service-Desk-articles/Winmail-dat-and-Jira-Service-Desk/ba-p/948080
Essentially, this happens because the sender is using a Microsoft mail client, like outlook. There are specific settings within that sender's email client that would have to be adjusted to prevent this. Jira does not have a means to change this, it is only storing the attached file in the manner it sees it. You could follow the steps in a guide such as https://www.lifewire.com/prevent-sending-winmail-dat-attachments-1173717 to have that client stop sending those kinds of .dat attachments.
I hope this helps.
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