code: @Nullable
    public String getPrimaryMimeTypeStringForExtension (@Nonnull final String sExtension)
    {
      ValueEnforcer.notNull (sExtension, "Extension");
  
      final ICommonsList <MimeTypeInfo> aInfos = getAllInfosOfExtension (sExtension);
      if (aInfos != null && aInfos.isNotEmpty ())
        return aInfos.getFirst ().getPrimaryMimeTypeString ();
      return null;
    }
docs: Get the primary (=first) mime type that is associated to the specified
  filename extension.
  
  @param sExtension
  The filename extension to search. May not be <code>null</code>.
  @return <code>null</code> if no mime type is associated with the passed
  extension.