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                the icon file extension.
            </td>
        </tr>
        <tr>
            <td>icons:{ placeholder }</td>
            <td>String</td>
            <td>
                '#em(<i>n</i>)_{alias}#'<br>
            </td>
            <td>
                该组表情插入文本框后的占位标识，必须包含'{alias}'部分，'{alias}'会被自动替换成每个表情的标识。<br>
                the placeholder for the icon group, must contains '{alias}' in it, it will be repla