Attachments= No[,Filter]

Attachments= Yes[,allowed_content_types[,Filter]]

Attachments= All[,allowed_content_types[,Filter]]

The Attachments= keyword controls LISTSERV's list-owner-configurable message attachment filter. This feature allows you to control the posting of various types of MIME attachments (images, audio, etc.) to your lists.  It is also possible to control the posting of inline uuencoded files to your lists on an on/off basis (off being the default if attachment control is enabled).

Note:  The ability of LISTSERV to filter or reject messages that contain MIME attachments is completely dependent on the ability of the poster's mail client to properly identify the MIME attachment when the mail is originally sent. 


Filtering/rejection is done based on the Content-Type headers found in the message--NOT by evaluation of the actual contents of the attachment. If for instance an executable binary (normally Content-Type: application/octet-stream) is sent by the client with a Content-Type of "text/plain", it will not be filtered or rejected by LISTSERV since (as noted below) text attachments are not covered by this keyword setting.

A registry of allowable MIME types for attachments, maintained by the Internet Assigned Numbers Authority (IANA) per RFC2048, can be found at

http://www.iana.org/assignments/media-types/index.html

A perhaps more easily-accessible list of MIME types pertaining to Microsoft Office 12-style documents (produced with Microsoft Office 2007 and later) is found at

https://technet.microsoft.com/en-us/library/ee309278%28office.12%29.aspx

The options are:

Attachments= Yes

All types of attachments are allowed to be posted to the list (the default).  Note however that other configuration options may still disallow the posting of certain attachments, and that "Attachments= Yes" does not override them.  For instance, if you have "Language= NoHTML", setting "Attachments= Yes" does not override the Language= setting.  Or if you have "Sizelim=" set to a value that precludes a file of x number of lines from being posted to the list, setting "Attachments= Yes" will not override the Sizelim= setting if the message with its attachment exceeds the number of lines specified by Sizelim=.



Attachments= No

All types of attachments are disallowed, other than plain text (always allowed) and HTML text (which is controlled exculsively by the "Language= NoHTML" keyword setting). With "Attachments= No", LISTSERV rejects messages containing attachments and bounces them back to the poster.



Attachments= No,Filter

Same as "Attachments= No", except that LISTSERV simply removes the unwanted material from the message and processes it instead of rejecting it out of hand. The removal of material is a silent operation and the poster is not notified that the attachment was discarded.


In all three of the above cases, when a message containing one or more uuencoded files is posted to the list, the encoded file(s) is/are stripped from the body of the message and the remainder of the message is passed through to the list.

Attachments= All

This setting tells LISTSERV to allow inline, uuencoded files, such as are generated by Microsoft Outlook, overriding the default.


One important restriction: UUencode filtering is strictly on/off. There is no attempt on the part of LISTSERV to guess file types when filtering is enabled (the default). This would be hazardous to begin with as support for these attachments is usually provided on a legacy basis in mail clients; that is, client A and client B could have a very different opinion on the type of the attachment.

It is also possible to allow certain MIME types to be passed through to the list while rejecting or filtering all others.  For instance,

Attachments= Yes,image,application/*msword


allows only the specified attachment types and rejects everything else. If you don't want to reject messages that contain other types of attachments, but just want to remove all other types of attachments, you add the ",Filter" parameter at the end of the line--ie,

Attachments= Yes,image,application/*msword,Filter


This means, "Allow all image and application/*msword attachments, and strip all other attachments".  Again, note that plain text ("Content-Type: text/plain") is always allowed and does not need to be included in the list of allowed attachment types.  Likewise, HTML text is controlled exclusively by the "Language= NoHTML" keyword setting.  Other text subtypes are, however, controlled by "Attachments=", so they need to be listed if you intend to allow them.

Additionally, should it be desired to allow all inline uuencoded files but restrict the list to certain MIME types, you can specify, similar to the above, something like

Attachments= All,image,application/*msword


or

Attachments= All,image,application/*msword,Filter


(In the preceding examples note carefully that "image" by itself is equivalent to "image/*"--in other words, when you code "Attachments= image", you are saying that all MIME image sub-types, for example, "image/jpeg", "image/gif", and so forth, are to be accepted.  If only certain sub-types are acceptable, for instance if you want to accept only JPEG graphics and ensure that others don't go through, you must specify the types explicitly--eg "Attachments= image/jpeg".)

Note carefully that simply coding something like "Attachments= image" will not necessarily allow all image files through. This is highly dependent on the client being used by the poster. For instance, if your client attaches all binary files as "Content-Type= application/octet-stream", regardless of whether a given binary is (for instance) an executable image, a Word file, or a compressed archive, and  you send a JPEG to a list with "Attachments= image" set in the header, it will be rejected since the image does not have a "Content-Type: image" tag. Most modern email clients will not exhibit this problem, however.

The rejection message sent by LISTSERV when ",Filter" is not specified is found in the BAD_ATTACHMENT mail template form. Note that the BAD_ATTACHMENT template form is a linear template and as such does not allow text formatting commands to be used.

The reason HTML text is not subject to "Attachments=" filtering is to allow you to reject (bounce) messages with attachments, while silently suppressing HTML text in multi-part messages which also contain a plain-text alternative.  Some mail programs send both HTML and plain-text versions of messages, and, even if you do not want HTML text on your list, there is little point in keeping out people who use it (who are often new to the Internet and aren't aware that their mail programs are sending HTML text) when you can simply remove the HTML part.  At the same time, you may want to reject postings containing images out of hand, rather than removing the images and continuing. The same applies to Exchange attachments, which are filtered by default (see "Language= Exchange").