For this issue, I suggest you follow these steps to troubleshoot the problem:
1. Send an email (with the same attachment) to users (on the same Exchange Server), then check whether they can see the attachment or not.
2. If they can see the attachment, this may client issue. I suggest you use another computer to check whether you can see the attachment or not.
3. If they also cannot see the attachment, please disable the AV software on your Exchange Server, then check for the attachment.
-I have seen a similiar issue, the user had recieved and opened so many of that file name over sometime that the temp folder was just at capacity with revision of that file and could not make anymore room. Once we backed up and deleted all the files in the temp folder they were able to receive and open attachments once again.