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="Boronia.JPG|992" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Baileyana.jpg" />
      <f ns="6" title="File:Basil.jpg" />
      <f ns="6" title="File:Bay.jpg" />
      <f ns="6" title="File:Betula lenta.jpg" />
      <f ns="6" title="File:Betula pendula.jpg" />
      <f ns="6" title="File:Bf0be7 b6cd5e94b2df4e4a978b58fd2823097a.jpg srz 248 248 85 22 0.50 1.20 0.jpg" />
      <f ns="6" title="File:Birch.jpg" />
      <f ns="6" title="File:Black pepper.jpg" />
      <f ns="6" title="File:Black spruce.jpg" />
      <f ns="6" title="File:Blackcurrant.jpg" />
    </allfileusages>
  </query>
</api>