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<body>
<input type="file" onchange="test(this,'txt');">
</body>
<script>
function test(obj,filter){
var file = obj.value.match(/[^\/\\]+$/gi)[0];
var rx = new RegExp('\\.(' + (filter?filter:'') + ')$','gi');
if(filter&&file&&!file.match(rx)){
alert("只能选择txt");
//重新构建input file
document.body.innerHTML = "<input type='file' onchange=\"test(this,'txt');\" >";
}
}
</script>