No, this is not the only thing. For some reason it was chosen to use octetstream instead of octect-stream ages ago. According to IANA, application/octectstream is not even a valid MIME type (Source: http://www.iana.org/assignments/media-types/application). This seems to be there in order to force compatibility with IE and Opera. Anyway, newer versions of these browsers should be capable of handling this octet-stream. The point of doing this is having a fallback to application/octetstream if no mimetype has been set. For the above reason, it seems like it was decided to use octetstream instead of octet-stream in order to have a better browser compatibility back in the ages.