You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query-continue>
    <allfileusages afcontinue="Gp.jpg|1153" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:BDM.jpg" />
      <f ns="6" title="File:Border map pic.png" />
      <f ns="6" title="File:Border scenic pic.png" />
      <f ns="6" title="File:Breacher.jpg" />
      <f ns="6" title="File:Bridge scenic pic.png" />
      <f ns="6" title="File:Bridge se scenic pic.png" />
      <f ns="6" title="File:Collapsed tunnel scenic pic.png" />
      <f ns="6" title="File:Dusk scenic pic.png" />
      <f ns="6" title="File:Flash.jpg" />
      <f ns="6" title="File:Fls scenic pic.png" />
    </allfileusages>
  </query>
</api>