两种办法:
1.
<html>
<head>
</head>
<body>
<form name="myform">
<input type="file" name="ff"/>
<input type="button" value="清空file中的内容" onclick="ff.select();document.execCommand('delete')" >
</form>
</body>
</html>
2.
<html>
<head>
</head>
<body>
<form name="myform">
<input type="file" id="f"/>
<input type="button" value="清空file中的内容" onclick="document.getElementById('f').outerHTML=document.getElementById('f').outerHTML">
</form>
</body>
</html>