No more error messages shown, once i uploaded a file of invalid size?
The error about invalid extension which you're seeing is invoked by File module in modules/file/file.js, here is a snapshot:

/**
 * File upload utility functions.
 */
Drupal.file = Drupal.file || {
  /**
   * Client-side file input validation of file extensions.
   */
  validateExtension: function (event) {
    // Remove any previous errors.
    $('.file-upload-js-error').remove();

    // Add client side validation for the input[type=file].
    var extensionPattern = event.data.extensions.replace(/,\s*/g, '|');
    if (extensionPattern.length &gt; 1 &amp;&amp; this.value.length &gt; 0) {
      var acceptableMatch = new RegExp('\\.(' + extensionPattern + ')$', 'gi');
      if (!acceptableMatch.test(this.value)) {
        var error = Drupal.t("The selected file %filename cannot be uploaded. Only files with the following extensions are allowed: %extensions.", {


Since it worked for the first time, but not on the second time, it's possible that you've some JS error which preventing this event to be invoked. If that's not the case, make sure file.js is included on the second time.

To debug that issue, you may create a breakpoint in above code to see if it's triggered properly. If it's not, using DevTools try enabling Event Listener Breakpoints for Control change/submit events, then try to reproduce the issue, so you'll know exactly what's happening.

Test also if you can reproduce the issue with File it-self using the default theme. If not, maybe it's worth to report a bug against that module.