When an email message is sent using Microsoft Outlook, chances are that its attachment will be displayed as WinMail.dat which requires specialized applications in order to be accessed.
One could easily resort to its native software, Outlook, to open it, but there are also some third-party apps that can be used.
Some of these tools can extract the attachment, whereas other will simply enable the users to preview the file's contents.
Even though some of these software solutions are standalone apps, one could also install add-ons for their email clients.