code: @Nullable
    public IMimeType getMimeTypeFromString (@Nullable final String s,
                                            @Nonnull final Charset aCharset,
                                            @Nullable final IMimeType aDefault)
    {
      return getMimeTypeFromBytes (s == null ? null : s.getBytes (aCharset), aDefault);
    }
docs: Try to find the MIME type that matches the passed content string.
  
  @param s
  The content string to check. May be <code>null</code>.
  @param aCharset
  The charset used to convert the string to a byte array. May not be
  <code>null</code>.
  @param aDefault
  The default MIME type to be returned, if no MIME type could be
  found. May be <code>null</code>.
  @return <code>aDefault</code> if no matching MIME type was found. May be
  <code>null</code>.