fix problem in firefox with empty file upload

lib/index.js

@@ -32,6 +32,14 @@ module.exports = function(options) {
                 file.on('end', function() {
                     if (!req.files)
                         req.files = {};
+
+
+                    // see: https://github.com/richardgirges/express-fileupload/issues/14
+                    // firefox uploads empty file in case of cache miss when f5ing page.
+                    // resulting in unexpected behavior. if there is no file data, the file is invalid.
+
+                    if(!buf.length) 
+                        return;
 
                     return req.files[fieldname] = {
                         name: filename,
@@ -50,6 +58,8 @@ module.exports = function(options) {
                             });
                         }
                     };
+
+
                 });
             });