Backload create ppt thumbnail issue

We are using Backload in our WebAPI project, Backload has a feature that creates thumbnails for each file you upload. This functionality works but when we are uploading a file with a .ppt extension I get an error, when I upload a file with the .pptx extension it works perfectly.

"Exception occured while storing the file"

We have a pretty basic web.config for Backload.

<backload xmlns:xsi="http://ift.tt/ra1lAU" xmlns:name="urn:backload-schema" xsi:noNamespaceSchemaLocation="Web.Backload.xsd" storageContext="Filesystem" enableIntegratedHandler="false">
   <fileSystem filesRoot="~/files" />
    <thumbnails width="24" height="24" dpi="96" />
  </backload>