<form action="/products/addCategory.jsp?action=submit&processFlowToken=<c:out value="${requestScope.processFlowToken}" />" method="post" enctype="multipart/form-data" name="frm_Product">

import org.apache.commons.fileupload.*;

   ...
   ...

   DiskFileUpload upload = new DiskFileUpload();

   if ( upload.isMultipartContent(req) )  {
      List items = upload.parseRequest(req);
                        
      Iterator iter = items.iterator();
      while (iter.hasNext()) {
         FileItem item = (FileItem) iter.next();

         if ( item.isFormField() ) {

            // process normal input form fields		
            System.out.println("Field: " + item.getFieldName() );
	    System.out.println("Value: " + item.getString() );

         } else  {

  	    // process uploaded fields.
            File fullFile = new File( item.getName() );

            // finding the physical directory the image is going to be stored at 
            String dirPath = getServletContext().getRealPath("/products/images/");

            // just my own little modification to the uploaded image filename.
            String fileName = trimImgFileName(fullFile.getName(), item.getFieldName(), nextId);

            // writing the image file
            item.write( new File( dirPath, fileName ) );
         }
      }
   }

   ...
   ...

    private String trimImgFileName(String fileName, String fieldName, String nextId) {
        StringBuffer sb = new StringBuffer(fileName);
        sb.replace(0, sb.lastIndexOf("."), nextId + "_" + fieldName);
        return sb.toString();
    }