XFOR: Attachment Names Changed in Messages To & From Internet

The Attachment names in messages received from Internet users are different from those from when the message was sent. For example, a message sent with an attachment named FILENAME.NEW, might be received with the attachment renamed to FILENAME.TXT.

This happens with messages sent using MIME encoding. The MIME header of each message received contains a field called "Content-Type". This field contains a value that indicates the type of the document. For example, this field could have a value of "text/plain" indicating that the attachment contains plain text.

RFC 1521 defines the following content types: New types and subtypes are defined by registering them with the Internet Assigned Numbers Authority (IANA).

The Microsoft Exchange Internet Mail Connector (IMC) uses the value of the Content-Type field to determine what the extension of the file name of the attachment should be. It determines the extension of the name based on the MIME Types mapping that is defined using the Microsoft Exchange administrator program.

You can define or modify the Content-Type Mappings in the MIME Types page of the Internet Mail Connector Properties dialog box in the Microsoft Exchange Administrator program. For Microsoft Exchange version 5.0, to view the MIME Types tab, go to the Protocols Object (Site or Server) and select File Properties, MIME Types will be the last Tab.

If the attachment name received has an extension and this extension is not what the Content Type mapping specifies, the IMC replaces the original extension with the new extension. If the attachment name received has no extension, the IMC adds an extension based on the content type header.

The benefit of having the IMC replace filename extensions is that is allows all attachments of a particular MIME Type to have the same extension when they are received by the client. For example, Microsoft Word documents (Content-Type: application/msword) always have an extension of .DOC regardless of what the extension was when the message was sent. This makes it easier for the client to open the attachment.

If you prefer that the IMC does not change the extensions of attachments of a particular content type, the entry in the IMC Content-Type Mapping for that content type should be removed. In this case, if the IMC does not find a mapping for a particular Content Type, it uses the filename received without modifying it.