MediaPlaceholder: add onFilesPreUpload property

[retrofox]

Description

The editor.MediaPlaceholder filter is great! and it allows us to customize the component easily. The only thing that I think it's missing is the ability to know what are the files that are being uploaded. For this, I've created this PR.
In short, you can know which files are going to be uploaded before to start the process to the server-side binding a function to the onFilesPreUpload property.

const customMediaPlaceholder = createHigherOrderComponent(
	CoreMediaPlaceholder => props => {
		const { children } = props;

		return (
			<CoreMediaPlaceholder
				{ ...props }
				onFilesPreUpload = { ( files ) => console.log( { files } ) }
			>
				{ children }
			</CoreMediaPlaceholder>
		);
	},
	'CustomMediaPlaceholder'
);

addFilter(
	'editor.MediaPlaceholder',
	'me/custom-media-placeholder,
	customMediaPlaceholder
);

How has this been tested?

It's a small change and doesn't modify the API of the component. Testing uploading a new file for a cover block could be a nice way to test that it doesn't break the functionality of the MediaPlaceholer, either uploading the file or dropping it.

Additionally, the example above is valid for testing purposes. Bind the function to the hook and confirm that the files list appears in the dev console in your dev client. In Chrome, for instance:

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• I've included developer documentation if appropriate.

[github-actions]

Size Change: +929 B (0%)

Total Size: 1.13 MB

Filename	Size	Change
build/a11y/index.js	1.14 kB	-1 B
build/block-editor/index.js	106 kB	-248 B (0%)
build/block-editor/style-rtl.css	11.8 kB	+374 B (3%)
build/block-editor/style.css	11.8 kB	+369 B (3%)
build/block-library/editor-rtl.css	7.88 kB	+14 B (0%)
build/block-library/editor.css	7.89 kB	+13 B (0%)
build/block-library/index.js	127 kB	+365 B (0%)
build/block-library/style-rtl.css	7.72 kB	+2 B (0%)
build/block-library/style.css	7.72 kB	+7 B (0%)
build/block-serialization-default-parser/index.js	1.88 kB	+1 B
build/blocks/index.js	48.1 kB	+1 B
build/components/index.js	194 kB	+22 B (0%)
build/components/style-rtl.css	19.5 kB	+9 B (0%)
build/components/style.css	19.5 kB	+7 B (0%)
build/core-data/index.js	11.4 kB	-3 B (0%)
build/data-controls/index.js	1.29 kB	-2 B (0%)
build/data/index.js	8.45 kB	-2 B (0%)
build/date/index.js	5.47 kB	-3 B (0%)
build/deprecated/index.js	772 B	+1 B
build/dom/index.js	3.17 kB	-1 B
build/edit-site/index.js	15.5 kB	+1 B
build/edit-widgets/index.js	9.34 kB	+1 B
build/is-shallow-equal/index.js	710 B	-1 B
build/keyboard-shortcuts/index.js	2.52 kB	+2 B (0%)
build/media-utils/index.js	5.3 kB	-1 B
build/plugins/index.js	2.56 kB	-1 B
build/token-list/index.js	1.28 kB	+1 B
build/url/index.js	4.06 kB	+2 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/annotations/index.js	3.62 kB	0 B
build/api-fetch/index.js	3.4 kB	0 B
build/autop/index.js	2.83 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.77 kB	0 B
build/block-directory/style-rtl.css	892 B	0 B
build/block-directory/style.css	892 B	0 B
build/block-library/theme-rtl.css	684 B	0 B
build/block-library/theme.css	686 B	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/compose/index.js	9.31 kB	0 B
build/dom-ready/index.js	569 B	0 B
build/edit-navigation/index.js	8.25 kB	0 B
build/edit-navigation/style-rtl.css	918 B	0 B
build/edit-navigation/style.css	919 B	0 B
build/edit-post/index.js	303 kB	0 B
build/edit-post/style-rtl.css	5.6 kB	0 B
build/edit-post/style.css	5.6 kB	0 B
build/edit-site/style-rtl.css	2.96 kB	0 B
build/edit-site/style.css	2.96 kB	0 B
build/edit-widgets/style-rtl.css	2.4 kB	0 B
build/edit-widgets/style.css	2.4 kB	0 B
build/editor/editor-styles-rtl.css	425 B	0 B
build/editor/editor-styles.css	428 B	0 B
build/editor/index.js	44.8 kB	0 B
build/editor/style-rtl.css	4.26 kB	0 B
build/editor/style.css	4.27 kB	0 B
build/element/index.js	4.65 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.72 kB	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	2.13 kB	0 B
build/html-entities/index.js	621 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.12 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.41 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/primitives/index.js	1.5 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/rich-text/index.js	14.8 kB	0 B
build/server-side-render/index.js	2.68 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/viewport/index.js	1.85 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

_{compressed-size-action}

[draganescu]

Tested and it breaks nothing!