To change the file extension for DITA output (not recommended):
[Setup] FileSuffix = .ext
The default file extension is .dita. Unless you have a compelling reason, use .dita, not .xml, for the output file extension. Many places in DITA depend on that extension, and will break if you do not use it; for example, related links would be assigned the wrong @format value, and so would not work correctly in many tools.