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 MediaWiki bug #2064 for details.

NOTE: 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).