Content Embellishments

There are a couple of other headers that MIME allows that increase functionality. These are:

· Content-Description: contains arbitrary text describing the content value.
· Content-Disposition: actually serves two purposes. First, it indicates whether a subordinate body part should be considered as an "inline content" or as an "attachment." In the former case, it should be processed with the rest of the message in sequence; in the latter case, it should be processed only when the user asks. For example, you might send a presentation to someone with some cover text. This could be sent as a multipart/mixed content type containing a text/plain and then some application-specific content value. The Content-Disposition: for the second value would mark it as an attachment.
Secondly, Content-Disposition: conveys file system semantics for the content value, for example, the corresponding file's name, creation time, and so on.
· Content-ID: contains a unique identifier for the content value, just as Message-ID: contains a unique identifier for a message.
· Content-MD5: contains an integrity checksum for the content value. It is meant to detect inadvertent changes to the content value. An interloper can still make changes to the content value and then recompute the Content-MD5: value.
· MIME-Version:, which always takes the value 1.0 - its mandatory presence and fixed value made for amusing discussion a decade ago.

Finally, there is an additional aspect of MIME that we've only touched on: character sets. [Further discussion of these is beyond the scope of this excerpt but can be found in Chapter 7 of the complete text. -Ed.]