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