A MIME attachment with the content type "application/octet-stream" is a
binary file. Typically, it will be an application or a document that must be
opened in an application, such as a spreadsheet or word processor. If the
attachment has a filename extension associated with it, you may be able to
tell what kind of file it is. A .exe extension, for example, indicates it is
a Windows or DOS program (executable), while a file ending in .doc is
probably meant to be opened in Microsoft Word.