code: @Nonnull
    public PreprocessorContext setExtensions(@Nonnull @MustNotContainNull final List<String> extensions) {
      this.extensions = new HashSet<>(assertDoesntContainNull(extensions));
      return this;
    }
docs: Set file extensions of files to be preprocessed, it is a comma separated list
  
  @param extensions comma separated extensions list of file extensions to be preprocessed, must not be null
  @return this preprocessor context