Change the extension of the generated files

The following configuration snippet would transform all files in the directory src/main/xml by applying the stylesheet src/main/stylesheet.xsl. The created files would have the extension .fo, rather than .xml.

  <build>
    <plugins>
      ...
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>xml-maven-plugin</artifactId>
        <executions>
          <execution>
            <goals>
              <goal>transform</goal>
            </goals>
          </execution>
        </executions>
        <configuration>
          <transformationSets>
            <transformationSet>
              <dir>src/main/xml</dir>
              <stylesheet>src/main/stylesheet.xsl</stylesheet>
              <fileMappers>
                <fileMapper implementation="org.codehaus.plexus.components.io.filemappers.FileExtensionMapper">
                  <targetExtension>.fo</targetExtension>
                </fileMapper>
              </fileMappers>
            </transformationSet>
          </transformationSets>
        </configuration>
      </plugin>
      ...
    </plugins>
  </build>

Btw, the same could be achieved the much more powerful and generic RegExpFileMapper, which you find explained in more detail in another example.