Manual:$wgMimeType

Output: $wgMimeType
The type to send in the Content-Type header.
Introduced in version:1.3.0
Removed in version:still in use
Allowed values:(string)
Default value:'text/html'

Details edit

The type to send in the Content-Type header of pages. Set to 'application/xhtml+xml' if you want pages to be treated as XHTML. Also 'application/xhtml+xml' is necessary to display MathML properly in Mozilla.