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="IMG_0008a1.jpg|2142" />
  </query-continue>
  <query>
    <allfileusages>
      <f fromid="1421" ns="6" title="File:Blue hills.jpg" />
      <f fromid="2137" ns="6" title="File:IMG 0002.jpg" />
      <f fromid="2137" ns="6" title="File:IMG 0003a1.jpg" />
      <f fromid="2137" ns="6" title="File:IMG 0003a2.jpg" />
      <f fromid="2137" ns="6" title="File:IMG 0004a1.jpg" />
      <f fromid="2137" ns="6" title="File:IMG 0004a2.jpg" />
      <f fromid="2137" ns="6" title="File:IMG 0005.jpg" />
      <f fromid="2142" ns="6" title="File:IMG 0006.jpg" />
      <f fromid="2142" ns="6" title="File:IMG 0007a1.jpg" />
      <f fromid="2142" ns="6" title="File:IMG 0007a2.jpg" />
    </allfileusages>
  </query>
</api>