MIME stands for Multipurpose Internet Mail Extensions, a standard developed by the Internet Engineering Task Force for email but then repurposed for the Web. Apache uses mod_mime.c, compiled in by default, to determine the type of a file from its extension. MIME types are more sophisticated than file extensions, providing a category (like “text,” “image,” or “application”), as well as a more specific identifier within that category. In addition to specifying the type of the file, MIME permits the specification of additional information, like the encoding used to represent characters.
The “type” of a file that is sent is indicated by a header near the beginning of the data. For instance:
content-type: text/html
indicates that what follows is to be treated as HTML, though it may also be treated as text. If the type were “image/jpg”, the browser would need to use a completely different bit of code to render the data.
This header is inserted automatically by Apache[1] based on the MIME type and is absorbed by the browser so you do not see it if you right-click in a browser window and select “View Source” (MSIE) or similar. Notwithstanding, it is an essential element of a web page.
The list of MIME types that Apache already knows about is distributed
in the file ..conf/mime.types or can be found at
http://www.isi.edu/in-notes/iana/assignments/media-types/media-types.
You can edit it to include extra types, or you can use the directives
discussed in this chapter. The default location for the file is
.../<site>/conf, but it may be more
convenient to keep it elsewhere, in which case you would use the
directive TypesConfig.
Changing the encoding of a file with one of these directives does not
change the value of the Last-Modified header, so
cached copies with the old label may linger after you make such
changes. (Servers often send a Last-Modified
header containing the date and time the content of was last changed,
so that the browser can use cached material at the other end if it is
still fresh.) Files can have more than one extension, and their order
normally doesn’t matter. If the extension
.itl maps onto Italian and
.html maps onto HTML, then the files
text.itl.html and text.html.itl
will be treated alike. However, any unrecognized
extension, say .xyz, wipes out all extensions to
its left. Hence text.itl.xyz.html will be
treated as HTML but not as Italian.
[1] If you are constructing HTML pages on the fly from CGI scripts, you have to insert it explicitly. See Chapter 14 for additional detail.