The file extension in configuration is only used to tell Pungi which
files from the task results should be downloaded. The user has to get it
right or the phase will fail. Each format has a single valid suffix.
Pungi should not require users to specify the suffix, since it can just
as well just know the right value.
The old configuration will continue working, only the extension will be
ignored.
Fixes: https://pagure.io/pungi/issue/753
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>