title: N: Ignoring file 'core' in directory '/etc/apt/sources.list.d/' as it has no filename extension
body: <p>when running apt-get update on Ubuntu 14.04 I get the following error.</p>
  
  <pre><code>N: Ignoring file 'core' in directory '/etc/apt/sources.list.d/' as it has no filename extension
  </code></pre>
  
  <p>This happened after I added a repository:</p>
  
  <pre><code>apt-add-repository ppa:duplicity-team/ppa
  apt-get update
  (...)   
  Segmentation fault (core dumped)
  </code></pre>
  
  <p>After this I couldn't update/upgrade packages, so I cleaned up my /etc/apt/sources.list.d/ and did a reboot. Now apt-get update is functional again but a file 'core' appeared in /etc/apt/sources.list.d/</p>
  
  <p>Is this file safe to delete? This is the content of the file:</p>
  
  <pre><code>^?ELF^B^A^A^@^@^@^@^@^@^@^@^@^D^@&gt;^@^A^@^@^@^@^@^@^@^@^@^@^@@^@^@^@^@^@^@^@^@^@$
  ^@^@^@^@^@^@�
  ^@^@^@^@^@^@^P^@^@^@^@^@^@^A^@^@^@^D^@^@^@^@^@^K^@^@^@^@^@^@@W^E�^?^@^@^@^@^@^@$
  ^G�^?^@^@^@^@^@^@^@^@^@^@^@ ^@^@^@^@^@^@^@ ^@^@^@^@^@^@^@^P^@^@^@^@^@^@^A^@^@^@$
  ^G�^?^@^@^@^@^@^@^@^@^@^@^@^P^@^@^@^@^@^@^@^P^@^@^@^@^@^@^@^P^@^@^@^@^@^@^A^@^@$
  ^G�^?^@^@^@^@^@^@^@^@^@^@^@^P^@^@^@^@^@^@^@P^T^@^@^@^@^@^@^P^@^@^@^@^@^@^A^@^@^$
  ^@^@^@^@^@^@^@^@^@^@^@^@^@^O�^C^@^@^@^@^@^P��^@^@^@^@^@^A^@^@^@^@^@^@^@@�Ѷ�^?^@$
  ^G�^?^@^@7^@^@^@^@^@^@^@^@�
  ^G�^?^@^@^@�
  ^G�^?^@^@6^@^@^@^@^@^@^@^@�
  ^G�^?^@^@^@�
  ^G�^?^@^@8^@^@^@^@^@^@^@^@�
  ^G�^?^@^@^@ ^_^G�^?^@^@^@^@^@^@^@^@^@^@^@ ^_^G�^?^@^@^@ ?^G�^?^@^@E^A^@^@^@^@^@$
  </code></pre>