MediaPlaceholder multiple and gallery properties (#9139)

* MediaPlaceholder multiple property

The `gallery` property should be set to `true` only after checking that the selected `type` of media is not `audio`.

* gallery and playlist check for multiple and type

gallery and playlist check for multiple and type, being gallery true when multiple is true and type is image, while playlist true when multiple is true and type is audio.

* type check when setting the gallery property

The `gallery` property is set to `true` when `multiple` is `true` and `type === "image"`

* double to single quotes

changed type check using single quotes instead of double quotes

packages/editor/src/components/media-placeholder/index.js

@@ -127,7 +127,7 @@ class MediaPlaceholder extends Component {
 					{ __( 'Upload' ) }
 				</FormFileUpload>
 				<MediaUpload
-					gallery={ multiple }
+					gallery={ multiple && type === 'image' }
 					multiple={ multiple }
 					onSelect={ onSelect }
 					type={ type }