code: public FileFilterBuilder addExtensions(Iterable<String> extensions) {
          for (String extension : extensions) {
              // Ultimately, SuffixFileFilter will be used, and the "." needs to be explicit.
              this.extensions.add(extension.startsWith(".") ? extension : "." + extension);
          }
          return this;
      }
docs: Add to the set of file extensions to accept for analysis. Case-insensitivity is assumed.
  
  @param extensions one or more file extensions to accept for analysis
  @return this builder