"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.Context.Binary(data) is the method which sends binary responses to the client. It accepts []byte value.