Manual:$wgJsMimeType

Details
Use this variable to set the media type (MIME type) which is used for scripts written in the JavaScript scripting language.

See RFC 4329 and [https://bugs.wikimedia.org/show_bug.cgi?id=2064 MediaWiki bug #2064] for details.

In theory, this should be set to   for all new wikis. Standard-compliant browsers support this setting. Internet Explorer from Microsoft does not support it in its current version (version 7).