Hi team, 

I would like to validate File name and extension of each files before upload the bulk files into BOX

 var uploader = new Box.ContentUploader();

 //shows upload UI element 
 function showUploadContainer() {
 
     uploader.show(folderId, ACCESS_TOKEN, {
         container: '.container'
     });          
 }

 uploader.on('beforeupload', function (data) {
     alert(dataa0].name);

// This popup works. But There is no way to stop firing or excluding the file in the uploader widgets. After this message, If I click upload, it uploads the file without any issues
     return false; (This is not working)

 });

when I upload the below files, I want to get the error message like you cannot upload as the file name has special characters (%) and file type not accepted

aaa.pdf

aaa.html

aa%a.docx