Background. That CUPS is a more security-aware printing system than traditional ones does not by default allow one to send deliberate (possibly binary) data to printing devices. (This could be easily abused to launch a Denial of Service attack on your printer(s), causing at least the loss of a lot of paper and ink.) “Unknown” data are regarded by CUPS as MIME type application/octet-stream . While you can send data “raw”, the MIME type for these must be one that is known to CUPS and allowed by it. The file /etc/cups/mime.types defines the “rules” of how CUPS recognizes MIME types. The file /etc/cups/mime.convs decides which file conversion filter(s) may be applied to which MIME types.