Clarification on jQuery ajax content-type

Question

What would be the difference/utility to use:

beforeSend: function(jqXHR) {
   jqXHR.overrideMimeType('text/plain; charset=iso-8859-1')
}

and contentType: 'text/plain; charset=iso-8859-1', contenttype just doesn’t work.

According to jQuery’s documentation, it should work.