src/it/projects/container-descriptors/custom-handler-withMixedEntries/assembly/src/assemble/bin.xml

<assembly xmlns="http://maven.apache.org/plugins/maven-assembly-plugin/assembly/1.1.0" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/plugins/maven-assembly-plugin/assembly/1.1.0 http://maven.apache.org/xsd/assembly-1.1.0.xsd">
  <id>bin</id>
  
  <formats>
    <format>dir</format>
  </formats>
  
  <includeBaseDirectory>false</includeBaseDirectory>
  
  <files>
    <file>
      <source>src/config/a/file.txt</source>
      <outputDirectory>a</outputDirectory>
      <destName>file.txt</destName>
    </file>
  </files>
  
  <fileSets>
    <fileSet>
      <directory>src/config/b</directory>
      <outputDirectory>b</outputDirectory>
    </fileSet>
  </fileSets>
  
  <containerDescriptorHandlers>
    <containerDescriptorHandler>
      <handlerName>file.txt</handlerName>
    </containerDescriptorHandler>
  </containerDescriptorHandlers>
</assembly>