public function setMimeType( $mimetype )
	{
		if ( $mimetype == $this->getMimeType() ) { return; }

		if( preg_match( '/^[a-z\-]+\/[a-zA-Z0-9\.\-\+]+$/', $mimetype ) !== 1 ) {
			throw new MShop_Media_Exception( sprintf( 'Invalid mime type "%1$s"', $mimetype ) );
		}

		$this->_values['mimetype'] = (string) $mimetype;
		$this->setModified();
	}