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>