The documentation is a bit rough regarding file transfers. I enabled the feature in the config file:
[..]
<param name=”enable-drive”>true</param>
<param name=”drive-path”>/var/guacamole/filetransfer/</param>
[..]
But it did not show me the Guacamole filesystem on my Windows client. Although the files were on the filesystem. The trick was to add guacdr-client.so to freerdp:
gua:~# mkdir /usr/lib/x86_64-linux-gnu/freerdp
gua:~# cp /usr/local/lib/freerdp/guacdr-client.so /usr/lib/x86_64-linux-gnu/freerdp/guacdr-client.so
gua:~# ll /usr/lib/x86_64-linux-gnu/freerdp/guacdr-client.so
-rwx—— 1 root root 612472 Jul 16 11:09 /usr/lib/x86_64-linux-gnu/freerdp/guacdr-client.so
gua:~# chmod og+rx /usr/lib/x86_64-linux-gnu/freerdp/guacdr-client.so
gua:~# chmod og+rx /usr/lib/x86_64-linux-gnu/freerdp/
gua:~# systemctl restart tomcat8