function shangchuan(){

var filepath = document.getElementById("file").value;
alert(filepath);
if(filepath==""){
alert("请选择上传的文件！");
document.getElementById("file").disabled=true;

}
var extname=filepath.substr(filepath.length-3, filepath.length);
alert("extname::"+extname);
//var extname = filepath.substring(filepath.lastIndexOf(".")+1,filepath.length);
// extname = extname.toLowerCase();//处理了大小写
if(extname!= "bmp" && extname!= "jpg" && extname!= "gif"){
alert("只能上传bmp,jpg,gif格式的图片！");

document.getElementById("file").disabled=true;
}
else{
document.getElementById("file").disabled=false;
}

}

<tr>
<td><input name="file12" style="width:200px; height: 100px;" type="file" id="file" ></td>
</tr>
<tr>
<td align="center"><button onclick="shangchuan()" >点击上传</button></td>
</tr>