Add new fields in "Files/Links Upload" tt_content - typo3-9.x

Question

I use File Links [uploads] Content Element to show list of files, and I need to add a field on this CE to show description.
I found this on the documentation : https://docs.typo3.org/m/typo3/reference-coreapi/8.7/en-us/ExtensionArchitecture/ExtendingTca/Examples/#example-2-extending-the-tt-content-table but I couldn't apply it because of a lack of skill with PHP and T3 Customization.
In which file I should add the follwing code :
CREATE TABLE tt_content (
tx_files_description tinyint(4) DEFAULT '0' NOT NULL
);
How can I customize the follwing code ? :
$temporaryColumn = array(
'tx_files_description' => array (
'exclude' => 0,
'label' =>
'LLL:EXT:examples/Resources/Private/Language/locallang_db.xlf:tt_content.tx_files_description',
'config' => array (
'type' => 'check',
)
)
);
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTCAcolumns(
'tt_content',
$temporaryColumn
);
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addFieldsToPalette(
'tt_content',
'visibility',
'tx_files_description',
'after:linkToTop'
);

Answer 1

Why do it in such a complicated way?
tt_contentrecords already has a lot of fields which are not used for rendering the CType uploads.
Even the description field is available, so it is no RTE field. (all CEs have this field for information in the backend only)
You only need to use it in the FE-rendering, though you need to modify the fluid template.
If you need a RTE field, you should activate the field bodytext in the BE form, as it comes with a proper definition and rendering.
You still have to insert the rendered field in the fluid template.

Answer 2

Since you created your own content element i can not really know how to position the lement, but what i can do is to help you create it. I just tested on my TYPO3 installtion and it works.
ext_tables.sql
CREATE TABLE tt_content (
tx_files_description text,
);
yourExtension/Configuration/TCA/Overrides/tt_content.php
$temporaryColumn = [
'tx_files_description' => [
'exclude' => true,
'label' => 'LLL:EXT:your_extension_key/Resources/Private/Language/locallang.xlf:tt_content.tx_files_description',
'config' => [
'type' => 'text',
'enableRichtext' => false,
],
],
];
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTCAcolumns(
'tt_content',
$temporaryColumn
);
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addToAllTCAtypes(
'tt_content',
'tx_files_description',
'general',
'before:media'
);
Assuming that you re using TYPO3 v9 go to the module Maintenance and press Analyze Database, then clear all cache.
If you are on TYPO3 v7-v8 then go to the install module and Run compare database (something like that). Clear the cache.
Then on your extended tab:
Best regards