13.6.2.2 Relabelling MIME Headers

Some user agents or gateways may emit messages with MIME headers that are less informative than they might be, but that nevertheless contain enough information to construct more precise MIME headers. Although the best solution is to properly configure such user agents or gateways, if they are not under your control, you can instead ask the MTA to try to reconstruct more useful MIME headers.

If the first probe of the CHARSET-CONVERSION mapping table yields a Yes or Always keyword, then the MTA will check for the presence of a conversions file. If a conversions file exists, then the MTA will look in it for an entry with RELABEL=1 and if it finds such an entry, the MTA will then perform any MIME relabelling specified in the entry. See 13.5.3 To Control Conversion Processing for information on conversions file entries.

For example, the combination of a CHARSET-CONVERSION table such as:

CHARSET-CONVERSION  

  IN-CHAN=tcp_local;OUT-CHAN=tcp_internal;CONVERT            Yes

and MTA conversion file entries of

out-chan=ims-ms; in-type=application; in-subtype=octet-stream; 
  in-parameter-name-0=name; in-parameter-value-0=*.ps; 
  out-type=application; out-subtype=postscript;   
  parameter-copy-0=*; relabel=1 

out-chan=ims-ms; in-type=application; in-subtype=octet-stream; 
  in-parameter-name-0=name; in-parameter-value-0=*.msw; 
  out-type=application; out-subtype=msword; 
     parameter-copy-0=* relabel=1

will result in messages that arrive on the tcp_local channel and are routed to the ims-ms channel, and that arrive originally with MIME labelling of application/octet-stream but have a filename parameter with the extension ps or msw, being relabelled as application/postscript or application/msword, respectively. (Note that this more precise labelling is what the original user agent or gateway should have performed itself.) Such a relabelling can be particularly useful in conjunction with a MIME-CONTENT-TYPES-TO-MR mapping table, used to convert such resulting MIME types back into appropriate MRTYPE tags, which needs precise MIME labelling in order to function optimally; if all content types were left labelled only as application/octet-stream, the MIME-CONTENT-TYPES-TO-MR mapping table could only, at best, unconditionally convert all such to one sort of MRTYPE.

With the above example and MIME-CONTENT-TYPES-TO-MR mapping table entries including

APPLICATION/POSTSCRIPT        PS 
APPLICATION/MSWORD              MW

a labelling coming in as, for example,

Content-type: application/octet-stream; name=stuff.ps

would be relabelled as

Content-type: application/postscript

and then converted into an MRTYPE tag PS to let Message Router know to expect PostScript.

Sometimes it is useful to do relabelling in the opposite sort of direction, “downgrading” specific MIME attachment labelling to application/octet-stream, the label for generic binary data. In particular, “downgrading” specific MIME labelling is often used in conjunction with the convert_octet_stream channel keyword on the mime_to_x400 channel (PMDF-X400) or xapi_local channel (PMDF-MB400) to force all binary MIME attachments to be converted to X.400 bodypart 14 format.

For instance, the combination of a CHARSET-CONVERSION mapping table such as

CHARSET-CONVERSION

    IN-CHAN=*;OUT-CHAN=mime_to_x400*;CONVERT Yes

and PMDF conversions file entries of

out-chan=mime_to_x400*; in-type=application; in-subtype=*;
   out-type=application; out-subtype=octet-stream; relabel=1
 
out-chan=mime_to_x400*; in-type=audio; in-subtype=*; 
   out-type=application; out-subtype=octet-stream; relabel=1 

out-chan=mime_to_x400*; in-type=image; in-subtype=*; 
   out-type=application; out-subtype=octet-stream; relabel=1 

out-chan=mime_to_x400*; in-type=video; in-subtype=*; 
   out-type=application; out-subtype=octet-stream; relabel=1

will result in downgrading various specific MIME attachment labelling to the generic application/octet-stream labelling (so that convert_octet_stream will apply) for all messages going to mime_to_x400* channels.