Manual:$wgUrlProtocols

Details
Defines the URL protocols that MediaWiki will recognize.

This is used in lots of places.

The most visible effect is it determines what protocols are allowed for external links: unrecognized protocols are ignored, no link is generated.

However, it's used in many other places too, including the code for Special:LinkSearch, and utility functions like  that are used in a wide variety of different places.

Some examples that don't work (because  is not defined as a valid protocol):

link text ]
 * test://www.example.com
 * [test://www.example.com]
 * [test://www.example.com

some browsers, like Mozilla Firefox, will not follow file URLs on pages that have been loaded via HTTP.

This is a security measure.

See this mozillaZine article for more information, including some hints on how to overcome this restriction.

most browsers do not support "remote file" URLs like file://host/share (only Internet Explorer handles these per default.

For Firefox, a plugin is required or use exactly 5 slashes like file://///host/share, see )

Modification (MW 1.6+)
To allow links to local files (which may make sense in an intranet context), put this into your LocalSettings.php:

Usually you only want to add protocols to this array.

The default protocols should all be safe to click on (no evil side effects), and removing a protocol from the list will cause URLs using those protocols to become unrecognized in many places throughout the software.

In particular, removing 'http://' or other common protocols will probably break huge amounts of stuff.

Nevertheless, if you need to do so (for example, you already have a News: namespace), you can do something like this:

The default value has changed in MediaWiki 1.6.0, before that version the following were not recognized by default:
 * telnet://
 * nntp://
 * worldwind://

The default value in MediaWiki 1.22.0 is

Modification (MW 1.5)
In earlier versions the setting was a regexp expression which matched the beginning part of the URI; in other words, it is a set of prefixes separated by '|' (with '/' converted to '\/' because '/' has a special meaning in a regular expression.

This default value causes MediaWiki to render URIs with the following prefixes as external links:
 * http://
 * https://
 * ftp://
 * irc://
 * gopher://
 * news:
 * mailto:

Older versions
If you need this functionality in older versions, there is the function  in ./includes/Parser.php.

Just add a line for your protocol there.