We have an Exchange 2013 environment, all users are on Outlook 2013 as well.
We just had an issue where a user replied to an email thread (from their Outlook client) and attached a .docx file
The email (according to the header) was sent in HTML format.
Multiple recipients of the email opened the email within their Outlook and could not see the attachment.
However they could see it on their phones.
Looking at the header of one of the recipients I could see 'winmail.dat' in there, which lead me to think it had something to do with HTML vs RTF format. However I can't find an issue with the sender.
I'm stuck at this point.