Settings->Configure Kate. In the left panel, select "Open/Save". Choose the "Modes Filetypes" tab and select "Markup/XML" from the dropdown menu at the top. Append ";*.XML" to the "File extensions" list. Click Apply and then OK.
Is this the method you were referring to? I think the developers may have moved it.
Try:
Settings->Configure Kate. In the left panel, select "Open/Save". Choose the "Modes Filetypes" tab and select "Markup/XML" from the dropdown menu at the top. Append ";*.XML" to the "File extensions" list. Click Apply and then OK.
Is this the method you were referring to? I think the developers may have moved it.