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Usually, it depends on one of the many gateways that an email passes through on it's path between sender and recipient. If that gateway has an incompatibility issue with the specific attachment format, it may convert it to the .dat and render it useless. It can also vary by recipient capability...you may be able to, for example, receive the same item to Outlook and can use the attachment just fine. It is quite variable.
What I suggest is testing with various senders...and see if any commonality can be discovered. Then work with the sender to work with their admins to try and resolve the issue.
For me, I have an issue when some emails are sent to a particular address of mine...which has an auto-forward over to my primary GMail account. In that path, attachments are indeed rendered useless...but the exact same email delivered directly to Gmail (not via the forward) comes through just fine. So it's that forwarding host that is the problem, in my case.
Occam's Razor nearly always applies when troubleshooting technology issues!
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