How to add a .xml extension to a specific URI using .htaccess

[Kalina]

I have a URL at the moment www.mydomain.com/feed/my_feed/ which writes out an xml document to be read by a third party. The XML document is dynamically generated (I'm using ExpressionEngine).The third party have requested that it must have a .xml extension to be read.Any ideas how I could set up a .htaccess rule to only use a .xml file extension for this particular uri?Most tutorials are concerned with removing file extensions adding file extensions to all urls, which doesn't help a .htaccess noob.Many thanks in advance.