code: public final Map<String, Set<String>> getExtensionMimeMap() throws OSException {
          initializeMimeExtensionArrays();
          return Collections.unmodifiableMap(extensionMime);
      }
docs: Creates a map with file extensions as keys pointing on sets of MIME types
  associated with them.
  
  @return Map of file extensions pointing on MIME types
  @throws OSException