MIME is not a type, but a protocol for identifying file types. BeOS uses MIME rather than file extensions at the OS-level to identify file types. MIME allows MacOS 9, which does not require file extensions, to send attachments to users of Windows (or Linux or BeOS or whatever) and have their computers correctly identify the file type and to associate the file with the correct application. Not all file types exist on all computers. For example, a Linux user who receives an AppleWorks file will be SOL. Does your mysterious attachment come from someone that you know? Is it possible for you to give us additional information about this attachment? Click Reply. Read everything in the quoted message. You may find some identifying information about your MIME attachment.