My site is on SharePoint 2013 and I have tried to change the MIME type for XML to both of the following:
xml application/xml
xml text/xml
Preferred result (just opens in the browser): https://www.apple.com/sitemap.xml
This link describes all of the ways for allowing a file to be viewed by the browser instead of downloading: http://howtosharepoint.blogspot.com/2011/04/open-pdf-files-in-browser.html
And also please check in which library your Xml is stored... in that library go to advanced library setting then check file opening option, there are few two few ddl like open in browser, open in client. Select open in browser and try again.... and also you can try deactivating file opening in client feature as well.