Css 如何限制文件名预览中选择上载的字符？

我正在使用twitter引导框架。 我单击“上载”按钮，然后选择需要上载的文件。然后“上载”按钮更改为“更改”，并在旁边显示所选文件的名称。但是，如果我们有一个大文件名，我希望将文件名字符限制在50个左右。如何实现这一点

<div class="span5">
        <div class="fileupload fileupload-new"
         data-provides="fileupload">
         <span class="btn btn-file" id="uploadRef"> <span class="fileupload-new">Upload
           Referral Script</span> <span class="fileupload-exists">Change</span>
          <form:input type="file" path="refuploadScript"
           onchange="return checkFileExists();" />
         </span> <span class="fileupload-preview"></span>


          <a href="#"
          class="close fileupload-exists" data-dismiss="fileupload"
          style="float: none">x</a>
        </div>
</div> 

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引用脚本更改

要执行此操作，请首先隐藏默认文件名显示范围。 然后处理文件名并单独显示

因此，要隐藏默认值：

<span class="fileupload-preview" style="display: none;"></span>

function truncatefile() // calling a function on change or select
{                           
   var fup = document.getElementById('patientupload'); //store file by ID
   var file = fup.files[0]; // store value of file
   var filePath = fup.value; 
   var name;                
   if(filePath=="")                 
   {                    
      document.getElementById('fname').innerHTML = '';                  
   }                
   else                 
   {                    
      name=filePath.replace(/^.*[\\\/]/, '');

      var ext = filePath.substring(filePath.lastIndexOf('.') + 1);//getting file extension

      var fileName = name.substring(0,name.length-4); // storing 0th position till extension begining

      var fileNameNew=""; 
      if(fileName.length > 30)  
      {
         var fileNameFst=fileName.substring(0,24); //firstpart of file
         var fileNameLst=fileName.substring(fileName.length-5,fileName.length); //last part of file
         fileNameNew=fileNameFst+ "...." + fileNameLst+"."+ext; //combine all parts         
      }         
      else
      {         
         fileNameNew=fileName+"."+ext; //if length less than 30 
      }                     
      document.getElementById('fname').innerHTML = fileNameNew;                 
   }