Liftweb provides AJAX functionality for all html elements except file upload using Scala .
But there is a way to upload file using AJAX by binding form as AJAX form .

1) add below lines in your html .

<div class="lift:FileSnippet.uploadFile">
Upload File : <input name="upload" type="file"></input>
<input  type="submit" name="submit" value="Upload"/>
</div>

2)

class FileSnippet {
      def uploadFile(form: NodeSeq): NodeSeq = {

          def handleFile():JsCmd = {
            // Add Your JavaScript Code
          }

          var fileHolder: Box[FileParamHolder] = Empty
          val bindForm =
            "type=file" #> SHtml.fileUpload((fph) => fileHolder = Full(fph)) &
            "type=submit" #> SHtml.ajaxSubmit("Submit", handleFile _)

         SHtml.ajaxForm(
           bindForm(form))
      }
}

You will get file element in fileHolder variable . In handleFile() , write your file functionality code .