Handle nested uploaded files correctly

[akrabat]

The $_FILES array is very weird when you have a nested name of the type:
<input name="files[details][avatar][]" type="file"> has the structure
is:

    [
        'files' => [
            'tmp_name' => [
                'details' => [
                    'avatar' => [
                        0 => __DIR__ . DIRECTORY_SEPARATOR . 'file0.txt',
                        1 => __DIR__ . DIRECTORY_SEPARATOR . 'file1.html',
                    ],
                ],
            ],
            'name' => [
                'details' => [
                    'avatar' => [
                        0 => 'file0.txt',
                        1 => 'file1.html',
                    ],
                ],
            ],
            'type' => [
                'details' => [
                    'avatar' => [
                        0 => 'text/plain',
                        1 => 'text/html',
                    ],
                ],
            ],
            'error' => [
                'details' => [
                    'avatar' => [
                        0 => 0,
                        1 => 0
                    ],
                ],
            ],
            'size' => [
                'details' => [
                    'avatar' => [
                        0 => 0,
                        1 => 0
                    ],
                ],
            ],
        ],
    ],

As such, when we iterate over the array, we have to normalise each time to ensure that keep the actual $_FILES array key names known all the way down to the actual data.

Fixes #1806

[weierophinney]

👍 This is the whole reason for the UploadedFileInterface. Of course, now we have to go and do errata to PSR-7, as evidently we documented the actual structure PHP does incorrectly.

[mathmarques]

👍