How to map files by extension in sbt?

Question

I want to compile WSDL files with the sbt-xjc plugin. By default, it looks for all the *.xsd files and I want to look for *.wsdl files. I tried this syntax:

sources in (Compile, xjc) ++= unmanagedResourceDirectories.value.flatMap(dirs => (dirs ** "*.wsdl").get)

But I get an error at sbt compile:

Reference to undefined setting: 

  *:unmanagedResourceDirectories from compile:xjc::sources (build.sbt:76)
     Did you mean web-assets:unmanagedResourceDirectories ?

How can I add all the *.wsdl files to xjc?