I could imagine that this is a follow-up problem of the solution discussed in this thread.
With 'mime' => 'xml' you are ending in a Content-Type header of text/xml, but I still think you should have application/xml. Unfortunately, as @sebastiangreger points out, you are bound using the mime options Kirby recognizes but there is no such option to produce application/xml.
However, you could try with 'mime' => 'xhtml' which will result in Content-Type application/xhtml+xml and I think this could work for you.