Multipurpose Internet Mail Extensions, or MIME, provides a means to interchange text in languages with different character sets. Multimedia email can be sent between different computer systems that use the SMTP protocol. MIME enables you to send and receive email messages containing:
Images
Sounds
Linux Tar Files
PostScript
FTP-able File Pointers
Non-ASCII Character Sets
Enriched Text
Nearly any other file
Because MIME handles such a variety of file types, you might need to customize aspects of MIME for your users.
An ASCII file called preamble.txt is installed in the GWIA gateway folder (domain\wpgate\gwia). This file, which is included with any MIME multipart message, is displayed when the message recipient lacks a MIME-compliant mail reader.
The content of the preamble.txt file is a warning, in English, that the file is being sent in MIME format. If the recipient cannot read the message, he or she needs to either use a MIME-compliant mail reader or reply to the sender and request the message not be sent in MIME format.
We recommend that you use the preamble.txt file so that those who read MIME messages coming from your GroupWise system and who lack MIME-compliant mail readers can understand why they cannot read the message and can take corrective action.
If you choose to modify the preamble.txt file, be aware of the following considerations:
The maximum file size is 1024 bytes (1 KB)
This file is read by the GWIA when the GWIA starts, so if you change the file, you must restart the GWIA.
The GWIA’s gateway folder also contains a preamble.all file. The preamble.all file includes the text of preamble.txt translated into several languages. If you anticipate that your users will be sending mail to non-English speaking users, you might want to copy the appropriate language sections from the preamble.all file to the preamble.txt file.
The 1024-byte limit on the size of the preamble.txt file still applies, so ensure that the file does not exceed 1024 bytes.
By default, the GroupWise client determines the MIME content-type and encoding for message attachments. If, for some reason, the GroupWise client cannot determine the appropriate MIME content-type and encoding for an attachment, the GWIA must determine the content-type and encoding.
The GWIA uses a mimetype.cfg file to map attachments to the appropriate MIME content types. Based on an attachment’s content type, the GWIA encodes the attachment using quoted-printable, Base64, or BinHex. Generally, quoted-printable is used for text-based files, Base64 for application files, and BinHex for Macintosh files.
The mimetype.cfg file includes mappings for many standard files. If necessary, you can modify the file to include additional mappings. If an attachment is sent that does not have a mapping in the file, the GWIA chooses quoted-printable, BinHex, or Base64 encoding.
The mimetype.cfg file is also used for RFC-822 attachments, but UUencode or BinHex encoding is used regardless of the mapped content type.
The mimetype.cfg file is located in the domain\wpgate\gwia folder. The following sections provide information you need to know to modify the file:
Each mapping entry in the file uses the following format:
content-type .ext|dtk-code|mac-ttttcccc [/parms] ["comment"]
|
Element |
Description |
|---|---|
|
content-type |
The MIME content type to which the file type is being mapped (for example, text/plain). You can omit the content-type only if you use the /parms element to explicitly define the encoding scheme for the file type. |
|
.ext|dtk-code|mac-ttttcccc |
The .ext element, dtk-code element, and mac-ttttcccc element are mutually exclusive. Each entry contains only one of the elements.
|
|
/parms |
Optional parameters that can be used to override the default encoding assigned to the MIME content type. Possible parameters are:
|
|
"comment" |
Optional content description |
The mimetype.cfg file contains the following four sections:
[Parameter-Override]
[Mac-Mappings]
[Detect-Mappings]
[Extension-Mappings]
The [Parameter-override] section takes priority over other sections. You can use this section to force the encoding scheme for certain file types. This section also contains defaults for sending various kinds of multipart messages. This is how the GWIA knows to put attachments into MIME Alternate/Parallel multiparts.
The [Mac-mappings] section defines mappings for Macintosh file attachments. The following is a sample entry:
application/msword mac-wdbnmswd "Word for Macintosh"
Macintosh files have a type and creator associated with them. The first four characters are used for the type and the last four characters are used for the creator application.
In the above example, the type is wdbn and the creator application is mswd. When a user attaches a Macintosh file to a message, the GWIA uses the appropriate entry in the [Map-mappings] section to map the file to a MIME content type and then encode the file according to the assigned encoding scheme. Unless otherwise specified by the /parms element, BinHex 4.0 is used for the encoding. The following example shows how you can use the /parms element to change the encoding from the default (BinHex) to Base64:
application/msword mac-wdbnmswd /base64 "Word for Macintosh"
If necessary, you can use ???? for the creator portion (mac-text????) to indicate a certain file type created by any application. Or, you can use ???? in both portions (mac-????????) to match any file type created by any application. For example:
application/octet-stream mac-???????? /base64 "Mac Files"
This causes all Macintosh files to be encoded using Base64 rather than BinHex.
GroupWise attempts to assign each attachment a detect code based on the attachment’s file type. The [Detect-mappings] section defines the mappings based on these detect codes. The following is a sample entry:
application/msword dtk-1000 "Microsoft Word 4"
The GWIA uses the detect code to map to a MIME content type and then encode the file according to the assigned encoding scheme. If there is no mapping specified or if the file type cannot be determined, one of the other mapping methods, such as Extension-Mappings, are used. The detect codes associated with attachments are GroupWise internal codes and cannot be changed.
If a mapping could not be made based on the entries in the [Mac-mappings] and [Detect-mappings] section, the GWIA uses the [Extension-mappings] section. The [Extension-mappings] section defines mappings based on the attachment’s file extension. The following is a sample entry:
application/pdf .pdf