Including example fails without a file extension (i.e., extensionless file)

Here are two example includes from the in-drafting Alpine User Handbook:

[source]
----
include::example$resolv.conf[]
----

[source]
----
include::example$dhcp_interfaces[]
----

The former properly expands. The latter results in the following output (in-html):

+include::example$dhcp_interfaces[]+

Both files exist. Renaming dhcp_interfaces to dhcp.interfaces and changing the _ in the include to a . resolves the issue.