FileCreationTrait for DTT how this should be done very solid.

FileCreationTrait.php

<?php

declare(strict_types=1);

namespace Drupal\Tests\server_general\Traits;

use Drupal\Core\File\FileExists;
use Drupal\file\FileInterface;
use weitzman\DrupalTestTraits\DrupalTrait;

/**
 * Provides file entity creation for tests using committed test assets.
 *
 * Assets live in tests/assets/ and are copied to public:// with
 * FileExists::Replace, so files never accumulate across test runs. The
 * physical file persists in public:// intentionally — this is safe for
 * parallel tests and avoids teardown race conditions.
 *
 * Available assets:
 * - test-image.png
 * - test-image.jpg
 * - test-file.pdf
 */
trait FileCreationTrait {

  use DrupalTrait;

  const ASSETS_PATH = 'modules/custom/server_general/tests/assets/';

  /**
   * Creates a File entity from a committed test asset.
   *
   * @param string $filename
   *   The asset filename (e.g. 'test-image.png'). Must exist in tests/assets/.
   *
   * @return \Drupal\file\FileInterface
   *   The saved File entity, marked for automatic cleanup.
   */
  protected function createFileEntity(string $filename): FileInterface {
    $source = \Drupal::root() . '/' . self::ASSETS_PATH . $filename;
    $uri = \Drupal::service('file_system')
      ->copy($source, 'public://' . $filename, FileExists::Replace);

    /** @var \Drupal\file\FileInterface $file */
    $file = \Drupal::entityTypeManager()
      ->getStorage('file')
      ->create([
        'uri' => $uri,
        'status' => 1,
      ]);
    $file->setPermanent();
    $file->save();
    $this->markEntityForCleanup($file);

    return $file;
  }

}

ServerGeneralFileCreationTraitTest.php

<?php

declare(strict_types=1);

namespace Drupal\Tests\server_general\ExistingSite;

use Drupal\Tests\server_general\Traits\FileCreationTrait;
use weitzman\DrupalTestTraits\Entity\MediaCreationTrait;

/**
 * Tests FileCreationTrait produces valid, clean File entities.
 */
class ServerGeneralFileCreationTraitTest extends ServerGeneralTestBase {

  use FileCreationTrait;
  use MediaCreationTrait;

  /**
   * Tests PNG file entity creation.
   */
  public function testCreatePngFileEntity(): void {
    $file = $this->createFileEntity('test-image.png');

    $this->assertNotEmpty($file->id());
    $this->assertEquals('public://test-image.png', $file->getFileUri());
    $this->assertEquals('image/png', $file->getMimeType());
    $this->assertTrue($file->isPermanent());
  }

  /**
   * Tests JPG file entity creation.
   */
  public function testCreateJpgFileEntity(): void {
    $file = $this->createFileEntity('test-image.jpg');

    $this->assertNotEmpty($file->id());
    $this->assertEquals('public://test-image.jpg', $file->getFileUri());
    $this->assertEquals('image/jpeg', $file->getMimeType());
    $this->assertTrue($file->isPermanent());
  }

  /**
   * Tests PDF file entity creation.
   */
  public function testCreatePdfFileEntity(): void {
    $file = $this->createFileEntity('test-file.pdf');

    $this->assertNotEmpty($file->id());
    $this->assertEquals('public://test-file.pdf', $file->getFileUri());
    $this->assertEquals('application/pdf', $file->getMimeType());
    $this->assertTrue($file->isPermanent());
  }

  /**
   * Tests creating a media image entity referencing a file entity.
   */
  public function testCreateMediaImage(): void {
    $file = $this->createFileEntity('test-image.png');

    $media = $this->createMedia([
      'bundle' => 'image',
      'name' => 'Test image',
      'field_media_image' => [
        'target_id' => $file->id(),
        'alt' => 'Test alt',
      ],
    ]);

    $this->assertNotEmpty($media->id());
    $this->assertEquals('Test image', $media->getName());
    $this->assertEquals($file->id(), $media->get('field_media_image')->target_id);
  }

  /**
   * Tests FileExists::Replace — calling twice produces same URI, no duplicates.
   */
  public function testFileExistsReplace(): void {
    $file1 = $this->createFileEntity('test-image.png');
    $file2 = $this->createFileEntity('test-image.png');

    $this->assertEquals($file1->getFileUri(), $file2->getFileUri());
    $this->assertNotEquals($file1->id(), $file2->id());
  }

}