I believe that i'm current.
In my test I called liquibase with the full path of the .sql file and that is what it wrote in the databasechangelog.
On my team we always set logicalFilePath= in the databasecahngelog element of our .xml files.
Many of our developers use staright sql for their conversions which is the motivation for experimenting with the .sql feature.
For the feature to become popular the following xml tag would need to work (and probably does) :
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<databaseChangeLog xmlns="http://www.liquibase.org/xml/ns/dbchangelog" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-3.1.xsd">
<include file="change1.xml" relativeToChangelogFile="true"/>
<include file="change2.sql" relativeToChangelogFile="true"/>
</databaseChangeLog>