Accept udt as extension in import document type

[bjarnef]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Similar to the fix in #11061 this also pass in invalidfiles to handleFiles function ... and further fix the accepted extension.
https://stackoverflow.com/questions/34128497/ngf-pattern-not-working-for-ng-file-upload/34128573#34128573

According to the docs https://github.com/danialfarid/ng-file-upload#full-reference ngf-change has these parameters:

ngf-change="upload($files, $file, $newFiles, $duplicateFiles, $invalidFiles, $event)"

Before

After

We can still switch to all files and select e.g. a start.svg, which would return an array on a single obj when logging to console:

$scope.handleFiles = function (files, event, invalidFiles) {
      console.log("handleFiles", files, event, invalidFiles);
      if (files && files.length > 0) {
          $scope.upload(files[0]);
      }
};

[umbrabot]

Hi there @bjarnef, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
• The level of security for this contribution is the same or improved
• The level of performance for this contribution is the same or improved
• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
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[nathanwoulfe]

Hey @bjarnef, this looks fine to me - system dialog defaults to .udt files, Umbraco provides UI feedback if I select an invalid file.

I've added a commit to fix the help text, which referenced a browse button as well as the import button, but only the latter exists.