13.6.2.3 MacMIME Format Conversions

Macintosh files have two parts, a resource fork that contains Macintosh specific information, and a data fork that contains data usable on other platforms. This introduces an additional complexity when transporting Macintosh files, as there are four different formats in common use for transporting the Macintosh file parts. Three of the formats, Applesingle, Binhex, and Macbinary, consist of the Macintosh resource fork and Macintosh data fork encoded together in one piece. The fourth format, Appledouble, is a multipart format with the resource fork and data fork in separate parts. Appledouble is hence the format most likely to be useful on non-Macintosh platforms, as in this case the resource fork part may be ignored and the data fork part is available for use by non-Macintosh applications. But the other formats may be useful when sending specifically to Macintoshes.

The MTA can convert between these various Macintosh formats. The CHARSET-CONVERSION keywords Appledouble, Applesingle, Binhex, or Macbinary tell the MTA to convert other MacMIME structured parts to a MIME structure of multipart/appledouble, application/applefile, application/mac-binhex40, or application/macbinary, respectively. Further, the Binhex or Macbinary keywords also request conversion to the specified format of non-MacMIME format parts that do nevertheless contain X-MAC-TYPE and X-MAC-CREATOR parameters on the MIME Content-type: header. The CHARSET-CONVERSION keyword Block tells the MTA to extract just the data fork from MacMIME format parts, discarding the resource fork; (since this loses information, use of Appledouble instead is generally preferable).

For example, the following CHARSET-CONVERSION table would tell the MTA to convert to Appledouble format when delivering to the VMS MAIL mailbox or a GroupWise postoffice, and to convert to Macbinary format when delivering to the Message Router channel:

CHARSET-CONVERSION
   IN-CHAN=*;OUT-CHAN=l;CONVERT              Appledouble 
   IN-CHAN=*;OUT-CHAN=wpo_local;CONVERT      Appledouble 
   IN-CHAN=*;OUT-CHAN=tcp_internal;CONVERT   Macbinary

The conversion to Appledouble format would only be applied to parts already in one of the MacMIME formats. The conversion to Macbinary format would only be applied to parts already in one of the MacMIME formats, or non-MacMIME parts which included X-MAC-TYPE and X-MAC-CREATOR parameters on the MIME Content-type: header.

When doing conversion to Appledouble or Block format, the MAC-TO-MIME-CONTENT-TYPES mapping table may be used to indicate what specific MIME label to put on the data fork of the Appledouble part, or the Block part, depending on what the Macintosh creator and Macintosh type information in the original Macintosh file were. Probes for this table have the form format|type|creator|filename where format is one of SINGLE, BINHEX or MACBINARY, where type and creator are the Macintosh type and Macintosh creator information in hex, respectively, and where filename is the filename.

For example, to convert to Appledouble when sending to the ims-ms channel and when doing so to use specific MIME labels for any MS Word or PostScript documents converted from MACBINARY or BINHEX parts, appropriate tables might be:

CHARSET-CONVERSION 

  IN-CHAN=*;OUT-CHAN=ims-ms;CONVERT     Appledouble


MAC-TO-MIME-CONTENT-TYPES 

! PostScript 
    MACBINARY|45505346|76677264|*     APPLICATION/POSTSCRIPT$Y 
    BINHEX|45505346|76677264|*        APPLICATION/POSTSCRIPT$Y 
! Microsoft Word 
    MACBINARY|5744424E|4D535744|*     APPLICATION/MSWORD$Y 
    BINHEX|5744424E|4D535744|*        APPLICATION/MSWORD$Y

Note that the template (right hand side) of the mapping entry must have the $Y flag set in order for the specified labelling to be performed. Sample entries for additional types of attachments may be found in the file mac_mappings.sample in the MTA table directory.

If you wish to convert non-MacMIME format parts to Binhex or Macbinary format, such parts need to have X-MAC-TYPE and X-MAC-CREATOR MIME Content-type: parameter values provided. Note that MIME relabelling can be used to force such parameters onto parts that would not otherwise have them.