Strange behavior of assembly

assembly.xml

<assembly>
    <id>assembly</id>
    <formats>
        <format>dir</format>
    </formats>
    <files>
        <file>
            <source>content.txt</source>
            <outputDirectory>/</outputDirectory>
            <destName>filtered-content.txt</destName>
            <filtered>true</filtered>
        </file>
    </files>
</assembly>

content.txt

some unfiltered text
foo expands to '${params.foo}'
bar expands to '${params.bar}'
"..." expands to '${...}'
"…" expands to '${…}'

pom.xml

<project>
    <modelVersion>4.0.0</modelVersion>
    <groupId>net.jsharp</groupId>
    <artifactId>assembly-example</artifactId>
    <version>1</version>
    <packaging>pom</packaging>
    <properties>
        <params.foo>FOO</params.foo>
        <params.bar>BAR</params.bar>
    </properties>
    <build>
        <pluginManagement>
            <plugins>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-assembly-plugin</artifactId>
                    <version>2.6</version>
                    <configuration>
                        <descriptors>
                            <descriptor>${project.basedir}/assembly.xml</descriptor>
                        </descriptors>
                    </configuration>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>
</project>

Unexpected output:

$ cat target/assembly-example-1-assembly/assembly-example-1/filtered-content.txt
some unfiltered text
foo expands to 'FOO'
bar expands to 'BAR'
"..." expands to 'MavenProject: net.jsharp:assembly-example:1 @ /path/to/my/code/pom.xml'
"…" expands to '${…}'