Bamboo Standalone ships with the Jetty application server (see Bamboo 'distributions'). To add additional MIME content types, edit the mimetypes.xml file in ../<Bamboo-install>/atlassian-bamboo/WEB-INF/mimetypes.xml and insert your new mime-mapping.

For instance to add an additional text/plain mime type with txt2 extension, insert the following:

<mime-mapping>
  <extension>txt2</extension>
  <mime-type>text/plain</mime-type>
</mime-mapping>

Restart Bamboo, for the changes to take effect. If you have any elastic agents running, ensure that they are shut down before you restart the Bamboo server. If you do not shut down your elastic instances before restarting, they will continue to run and become orphaned from your Bamboo server.