I have a very strange behaviour in Outlook (web, for Windows or for Mac) when I receive a mail with an attachment. The email is sent by a backend (Microsoft BizTalk) and Mime properties are set accordingly to set the file name of the attachment.
When the email is received in Outlook for Windows, the attachment name is displayed "body.csv" and when I download it, the name of the file is renamed according to the mime property set by the sender. In Gmail, the filename of the attachment is displayed successfully.
I don't know if this is a known bug in Outlook or a mistake in the sender code while setting the MIME properties.