In the web.config file at that location, something like:
<mimeMap fileExtension=".xml" mimeType="text/x-cross-domain-policy" />
Would be my guess.
(Tales From Behind The Curtain)
That's a slight variation on what the UI does, which is worth explaining because it's not easily discoverable - to edit a setting for a particular file but not the folder, you can go to the Content view tab in that folder, then pick the file you're interested in, right-click it and choose Features View. That targets just that file (as can be confirmed in the pane at the left, and in the status bar when you're editing a feature).
It Removed and Add -ed the type, but I find that fragile, so I put a Clear in there instead.