If you are using Dropbox.js for file uploading and wants to upload some specific extension type like csv, xls etc. Please follow the below code::

Dropzone.options.uploadcontact = {
    maxFilesize: 10, // Mb
    acceptedFiles: ".xls,.xlsx,.csv", // accepted file type
    maxFiles: 1, 
    init: function () { 

        // Set up any event handlers
        this.on('complete', function (file,done) {
            if (this.getUploadingFiles().length === 0 && this.getQueuedFiles().length === 0) {
              if (this.files[1]!=null){
              this.removeFile(this.files[0]);
              }
                if (this.getRejectedFiles().length === 1) {
                //alert("You can't provide both 'acceptedFiles' and 'acceptedMimeTypes'. 'acceptedMimeTypes' is deprecated.");
                }else{ window.location.href = "list-uploaded-contact"; // form action 
              } 

            }
        });
    }
};