Hi Folks,
Many, many thanks for
clearing up my mistakes. I have revised the summary based upon your
comments. Please let me know of any remaining errors. /Roger
A Summary of XML and Media Types (MIME)
What is XML’s MIME Type?
At this URL is a list of the 350 different MIME types:
http://www.iana.org/assignments/media-types/
In this list you will see two different MIME types for
XML:
application/xml
text/xml
The later MIME type (text/xml) has been deprecated. Thus, the official
MIME type for XML is:
application/xml
Note the format for expressing MIME types – it
contains two parts, separated by a slash:
type/subtype
What is Purpose of a MIME Type?
Suppose a browser, Web server, or other application is presented
with a resource (document). The purpose of a MIME type is to give information to
the browser, Web server, or other application about the format of the data
contained within the resource.
Where Does a MIME Type Come From?
MIME types are metadata. A MIME type is not stored
within a resource. It is not stored as a property of a resource. Heuristics
are used for determining the MIME type of a resource. In other words, a system
“guesses” what the MIME type is.
On Windows the MIME type is guessed by using the file
extension. In the Window’s Registry is a mapping from file
extension to MIME type.
Examples:
- If a file ends with the
extension .txt then the MIME type of the file is guessed to be text/plain.
- If a file ends with the
extension .doc then the MIME type of the file is guessed to be application/msword.
- If a file ends with the
extension .xml then the MIME type of the file is guessed to be application/xml.
- If a file ends with the
extension .zip then the MIME type of the file is guessed to be application/zip.
Can a MIME Type be Wrong?
Yes! You could create a Word document and deliberately
give the document a false extension, such as “.xml”. Windows
will then guess that the MIME type for the document is application/xml, which is clearly incorrect.
Why is MIME Type Important?
It would appear that MIME types are redundant. After
all a file extension can tell you what kind of data a file contains,
right? True. However, when you send a file across the Web, a file
looses its file extension, only the contents are sent, not the filename (or
file extension). This is where MIME becomes important. When data is
sent across the Web, it is sent as the payload of an HTTP message. In the
HTTP header is a field called Content-type, and the value of this field is a
MIME type, e.g.,
Content-type: application/xml
Thus, when a Web server receives data it examines the
Content-type header field to determine the type of data that is in the payload.
What is an XML Document?
Take this simple XML:
<?xml version="1.0"?>
<root>
Blah
</root>
and put it into Word and give the document the
extension “.xml”. As we’ve seen above the MIME type is:
application/xml
However, it is not an XML document. It is a Word document.
Conversely, take the same XML and put it into Notepad and
give the document the extension “.txt”. As we’ve seen the
MIME type is:
text/plain
Yet, it is an XML document.
So, what is an XML document? Answer: an XML document
is one that has an XML declaration as the first thing in the file. If you
open the above Word
document you won’t find an XML declaration as the first thing. If
you open the above Notepad
document you will find an XML declaration as the first thing.
Further Information
Elliotte Rusty Harold has written an excellent article on
this subject:
http://www-128.ibm.com/developerworks/xml/library/x-mxd2.html
Acknowledgements
I would like to gratefully acknowledge the excellent
inputs from these people:
Mitch Amiano
Rick Jelliffe
Amelia Lewis
Rick Marshall
Dave Pawson
Bryan Rasmussen
Henri Sivonen
Nathan Young
