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="BREADBOARD_POWER_SUPPLY_MODULE.jpg" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:B.png" />
      <f ns="6" title="File:BL18 ECG Maven-01.jpg" />
      <f ns="6" title="File:BL18 ECG Maven hardware.jpg" />
      <f ns="6" title="File:BLE shield.jpg" />
      <f ns="6" title="File:BLE shield1.jpg" />
      <f ns="6" title="File:BLE shield2.jpg" />
      <f ns="6" title="File:BLE shield3.jpg" />
      <f ns="6" title="File:BLE shield4.jpg" />
      <f ns="6" title="File:BLE shield5.jpg" />
      <f ns="6" title="File:BMP085.jpg" />
    </allfileusages>
  </query>
</api>