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.
No matter what kind of file it is, an application/octet-stream attachment is rarely viewable in an email, Usenet, or web client.
In addition to the generic application/octet-stream content type, you may also encounter attachments that have different subtypes (e.g., application/postscript, application/x-macbinary, and application-msword). They are similar to application/octet-stream, but apply to specific kinds of files.