Answering my own question, I`ve found two ways:
1. Find and edit\ monkey-patch the file application_config.rb of the gem itself.(I have vendorized the gem.)
2. Override the configs in a usual way, like the gem creators advise.
The config itself should be like this:
:photo => {
:missing_thumb => 'icon_missing_thumb.gif',
:missing_medium => "icon_missing_medium.gif",
:paperclip_options => {
storage: 's3',
s3_credentials: {
use_persistent: true,
:bucket => "bucket_name",
:endpoint => 'bucket_name.s3.amazonaws.com',
:access_key_id => "key",
:secret_access_key => "secret_key",
s3_host_name: "host_name"},
:default_url => "",
I`ve added the s3_credentials section and storage: 's3' line.