UNKNOWN

Special marker mimetype for unknown or unspecified mime types. These will generally be treated as TEXT for editor purposes. (The standard "unknown" mime type is application/octet-stream (from RFC 2046) but we know this isn't binary data; it's text.)

Note that MimeType is generally nullable in places where it's optional instead of being set to this value, but this mime type is there for places where we need a specific value to point to.