TYPO3 Core

Patch set 2 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/83646

For reference, here's how you could already achieve this with the AfterFlexFormDataStructureParsedEvent:

In a custom extension (i.e. sitepackage, with a vendor like "MyVendor") do this:

1. Add to sitepackage/Configuration/Services.yaml:

services:
    _defaults:
        autowire: true
        autoconfigure: true
        public: false

        MyVendor\Sitepackage\:
            resource: '../Classes/*'

        MyVendor\Sitepackage\Backend\FlexFormParsingModifyEventListener:
            tags:
                - name: event.listener
                  identifier: 'sitepackage/modify-data-structure'
                  method: 'modifyDataStructure'

2. Create sitepackage/Classes/Backend/FlexFormParsingModifyEventListener.php:

<?php
declare(strict_types=1);

namespace MyVendor\Sitepackage\Backend;

use TYPO3\CMS\Core\Configuration\Event\AfterFlexFormDataStructureParsedEvent;

final class FlexFormParsingModifyEventListener
{
    public function modifyDataStructure(AfterFlexFormDataStructureParsedEvent $event): void
    {
        $identifier = $event->getIdentifier();
        if (
            (($identifier['fieldName'] ?? '') === 'flex_file_1') &&
            (($identifier['tableName'] ?? '') === 'tx_styleguide_flex') &&
            (($identifier['dataStructureKey'] ?? '') === 'default')
        ) {
            $parsedDataStructure = $event->getDataStructure();
            if (isset($parsedDataStructure['sheets']['sDEF']['ROOT']['el']['input_1']['config'])) {
                $parsedDataStructure['sheets']['sDEF']['ROOT']['el']['input_1']['config']['max'] = 30;
                $parsedDataStructure['sheets']['sDEF']['ROOT']['el']['input_1']['config']['size'] = 5;
            }
            $event->setDataStructure($parsedDataStructure);
        }
    }
}

3. Reset your cache, go to the backend, and it should now populate input_1 with the extra attributes.

Note that this PSR-14 event is much more flexibel because the Matrix is not enforced here, and thus you can perform any change you see fit (also changing input types). However, the TCEFORM TSconfig way is less hassle to implement.

Patch set 3 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/83646

Patch set 4 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/83646

Patch set 5 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/83646

Patch set 6 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/83646

Patch set 7 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/83646

Patch set 8 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/83646

Patch set 9 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/83646

Patch set 10 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/83646