"The Selected file was not found” Error When Adding an Attachment to an InfoPath Form

Adding the following bit of JavaScript to the WrkTaskIP.aspx page, which is responsible for hosting the InfoPath task form:

<script type="text/javascript">   
 aspnetForm.encoding = "multipart/form-data";
 </script>

Another approach documented here suggests modifying the application.master master page file to add the necessary enctype form tag:

<form runat="server" onsubmit="return _spFormOnSubmitWrapper();" 
 enctype="multipart/form-data">    ...  </form