Extending File: can't add field to Root.Main - php

Question

I want to add a Tag field to all Files in a SilverStripe 3.3.1 site. I'm using Blog v2.4.0 and have upgraded tagfield to v1.2.1.
My FileExtension is configured to extend File:
class FileExtension extends DataExtension
{
private static $many_many = ['FileTags' => 'FileTag'];
public function updateCMSFields(FieldList $fields)
{
$tagField = TagField::create('FileTags', 'Tags', FileTag::get(), $this->owner->FileTags())
->setShouldLazyLoad(true)
->setCanCreate(true);
$fields->push($tagField);
}
}
The FileTag class is:
class FileTag extends DataObject
{
private static $db = ['Title' => 'Varchar(255)'];
private static $belongs_many_many = ['Files' => 'File'];
}
The extension hook I'm attaching to is here, and it provides a standard FieldList as described in the extension documentation.
The problem!
The field shows up correctly, but it's outside of the Root.Main tab and looks like this:
I've tried:
Using addFieldToTab('Root.Main', $field) - it results in [User Error] FieldList::addFieldToTab() Tried to add a tab to object 'FieldList' - 'Root' didn't exist.
As above, using Root and Main
Why doesn't this work the way it is supposed to?

Answer 1

The problem is Folder is a type of file, but Folder does not have a Root.Main tab in its CMS FieldList.
When adding fields to File we need to check that the File type is not Folder:
class FileExtension extends DataExtension {
public function updateCMSFields(FieldList $fields) {
if (!$this->owner instanceof Folder) {
$tagField = TagField::create('FileTags', 'Tags', FileTag::get(), $this->owner->FileTags())
->setShouldLazyLoad(true)
->setCanCreate(true);
$fields->addFieldToTab('Root.Main', $tagField);
}
}
}