How to test HTTP attach() with a faker uploaded file

I have a method on a controller that receive a file upload and then retransmits via HTTP Client to an external post API route. It works but I'm unable to test it properly: when I retrive to get the fake storage file (with Storage::get()) I obtain an empty string and so the attach() method on the HTTP call fails with this error: InvalidArgumentException: A 'contents' key is required

This is my controller's method:

use Illuminate\Support\Facades\Http;
use Illuminate\Support\Facades\Storage;
use App\Http\Requests\StoreRequest;

public function store(StoreRequest $request) {
    $data = [
        'name' => $request->name,
        // ...
    ];
    
    $response = Http::withToken( $this->token );

    if ($request->hasFile('file')) {
        $file = date('Ymd') . '-file.' . $request->file->extension();
        $request->file->storeAs('files', $file);

        if (Storage::exists("files/$file")) {
            dump(Storage::get("files/$file")); // -> enter on if but empty string on test
            $response = $response->attach('file', Storage::get("files/$file"), $file);
        }
    }

    $response = $response->post($this->endpoint, $data);

    if ($response->successful()) {
        return redirect()->route('thankyou');
    }
}

and this is my test:

use Illuminate\Http\UploadedFile;
use Illuminate\Support\Facades\Http;
use Illuminate\Support\Facades\Storage;

/** @test */
public function check_that_store_works()
{
    Storage::fake();
    Http::fake();

    $data = [
        'name' => $this->faker->firstName,
        'file' => UploadedFile::fake()->create('file.pdf'),
    ];
    $file = date('Ymd') . '-file.pdf';

    $this->post( route('files.store'), $data )
        ->assertRedirect( route('thankyou') );

    Storage::assertExists( "files/$file" );
}