The Content-Type header field describes the MIME type of the entity body. The MIME type is a standardized name that describes the underlying type of media carried as cargo (HTML file, Microsoft Word document, MPEG video, etc.). Client applications use the MIME type to properly decipher and process the content.

In the case of the HEAD request, Content-Type shows the type that would have been sent if it was a GET request.

The Content-Type values are standardized MIME types, registered with the Internet Assigned Numbers Authority (IANA). MIME types consist of a primary media type (e.g., text, image, audio), followed by a slash, followed by a subtype that further specifies the media type. Table 15-1 lists a few common MIME types for the Content-Type header. More MIME types are listed in Appendix D.

Table 15-1. Common media types

Media type Description
text/html Entity body is an HTML document
text/plain Entity body is a document in plain text
image/gif Entity body is an image of type GIF
image/jpeg Entity body is an image of type JPEG
audio/x-wav Entity body contains WAV sound data
model/vrml Entity body is a three-dimensional VRML model
application/vnd.ms-powerpoint Entity body is a Microsoft PowerPoint presentation
multipart/byteranges Entity body has multiple parts, each containing a different range (in bytes) of the full document
message/http Entity body contains a complete HTTP message (see TRACE)

It is important to note that the Content-Type header specifies the media type of the original entity body. If the entity has gone through content encoding, for example, the Content-Type header will still specify the entity body type before the encoding.

Character Encodings for Text Media

The Content-Type header also supports optional parameters to further specify the content type. The "charset" parameter is the primary example, specifying the mechanism to convert bits from the entity into characters in a text file: 

Content-Type: text/html; charset=iso-8859-4

We talk about character sets in detail in Chapter 16.

Multipart Media Types

MIME "multipart" email messages contain multiple messages stuck together and sent as a single, complex message. Each component is self-contained, with its own set of headers describing its content; the different components are concatenated together and delimited by a string.

HTTP also supports multipart bodies; however, they typically are sent in only one of two situations: in fill-in form submissions and in range responses carrying pieces of a document.

Multipart Form Submissions

When an HTTP fill-in form is submitted, variable-length text fields and uploaded objects are sent as separate parts of a multipart body, allowing forms to be filled out with values of different types and lengths. For example, you may choose to fill out a form that asks for your name and a description with your nickname and a small photo, while your friend may put down her full name and a long essay describing her passion for fixing Volkswagen buses.

HTTP sends such requests with a Content-Type: multipart/form-data header or a Content-Type: multipart/mixed header and a multipart body, like this: 

Content-Type: multipart/form-data; boundary=[abcdefghijklmnopqrstuvwxyz]

where the boundary specifies the delimiter string between the different parts of the body.

The following example illustrates multipart/form-data encoding. Suppose we have this form: 

<FORM action="http://server.com/cgi/handle"
 enctype="multipart/form-data"
 method="post">
<P>
What is your name? <INPUT type="text" name="submit-name"><BR>
What files are you sending? <INPUT type="file" name="files"><BR>
<INPUT type="submit" value="Send"> <INPUT type="reset">
</FORM>

If the user enters "Sally" in the text-input field and selects the text file "essayfile.txt," the user agent might send back the following data: 

Content-Type: multipart/form-data; boundary=AaB03x
--AaB03x
Content-Disposition: form-data; name="submit-name"
Sally
--AaB03x
Content-Disposition: form-data; name="files"; filename="essayfile.txt"
Content-Type: text/plain
...contents of essayfile.txt...
--AaB03x--

If the user selected a second (image) file, "imagefile.gif," the user agent might construct the parts as follows: 

Content-Type: multipart/form-data; boundary=AaB03x
--AaB03x
Content-Disposition: form-data; name="submit-name"
Sally
--AaB03x
Content-Disposition: form-data; name="files"
Content-Type: multipart/mixed; boundary=BbC04y
--BbC04y
Content-Disposition: file; filename="essayfile.txt"
Content-Type: text/plain
...contents of essayfile.txt...
--BbC04y
Content-Disposition: file; filename="imagefile.gif"
Content-Type: image/gif
Content-Transfer-Encoding: binary
...contents of imagefile.gif...
--BbC04y--
--AaB03x--

Multipart Range Responses

HTTP responses to range requests also can be multipart. Such responses come with a Content-Type: multipart/byteranges header and a multipart body with the different ranges. Here is an example of a multipart response to a request for different ranges of a document: 

HTTP/1.0 206 Partial content
Server: Microsoft-IIS/5.0
Date: Sun, 10 Dec 2000 19:11:20 GMT
Content-Location: http://www.joes-hardware.com/gettysburg.txt
Content-Type: multipart/x-byteranges; boundary=--[abcdefghijklmnopqrstuvwxyz]--
Last-Modified: Sat, 09 Dec 2000 00:38:47 GMT
 
--[abcdefghijklmnopqrstuvwxyz]--
Content-Type: text/plain
Content-Range: bytes 0-174/1441
 
Fourscore and seven years ago our fathers brough forth on this continent
a new nation, conceived in liberty and dedicated to the proposition that
all men are created equal.
--[abcdefghijklmnopqrstuvwxyz]--
Content-Type: text/plain
Content-Range: bytes 552-761/1441
 
But in a larger sense, we can not dedicate, we can not consecrate,
we can not hallow this ground. The brave men, living and dead who
struggled here have consecrated it far above our poor power to add
or detract.
--[abcdefghijklmnopqrstuvwxyz]--
Content-Type: text/plain
Content-Range: bytes 1344-1441/1441
 
and that government of the people, by the people, for the people shall
not perish from the earth.
 
--[abcdefghijklmnopqrstuvwxyz]--

Range requests are discussed in more detail later in this chapter.

 

Hypertext Transfer Protocol (HTTP)