code: @Pure
  	public static File addExtension(File filename, String extension) {
  		if (filename != null && !hasExtension(filename, extension)) {
  			String extent = extension;
  			if (!"".equals(extent) && !extent.startsWith(EXTENSION_SEPARATOR)) { //$NON-NLS-1$
  				extent = EXTENSION_SEPARATOR + extent;
  			}
  			return new File(filename.getParentFile(), filename.getName() + extent);
  		}
  		return filename;
  	}
docs: Add the extension of to specified filename.
  If the filename has already the given extension, the filename is not changed.
  If the filename has no extension or an other extension, the specified one is added.
  
  @param filename is the filename to parse.
  @param extension is the extension to remove if it is existing.
  @return the filename with the extension.
  @since 6.0