web.xml

In order to enable multipart requests (e.g. image upload), one needs to enable the upload option within the file .../webapps/lenya/WEB-INF/web.xml

<init-param>
  <param-name>enable-uploads</param-name>
  <param-value>true</param-value>
</init-param>

Please restart your servlet engine after editing web.xml.

If one builds Lenya from source then multipart requests can be enabled by editing the file build.properties (enable.uploads=true).