A few general rules for the MIME types are:
The x- prefix of a MIME subtype-identifier implies that it is non-standard i.e. not registered with IANA. e.g. Adobe Flash: application/x-shockwave-flash
The vnd prefix of a MIME subtype-identifier means that the MIME value is vendor specific. e.g. Microsoft Excel: application/vnd.ms-excel
MIME type for unknown file type is generally application/octet-stream.
https://www.nih-cfde.org/resource/intro-mime-type/