The private-key file of a file-server is ignored in the installation servlet
A file-server item in BASE can be configured in various ways. The simplest being username+password, but it is also possible to use a private-key file (http://base.thep.lu.se/ticket/2112).
File-server items used by Reggie can be configured to use a private-key file and it seems to work for accessing files, but the installation servlet complains:
sftp://....: Exhausted available authentication methods