docs: adds documentation for "fileUploadPreview"

Author: iOvergaard

16/umbraco-cms/customizing/extending-overview/extension-types/README.md

@@ -135,6 +135,7 @@ These are the current types of UI Extensions:
 | entityAction               | An entity action is a button that can be added to any entity, like a document, media, member, etc. It will be shown in the entity actions menu and in the entity actions menu.                                                                                                             |
 | entityBulkAction           | An entity bulk action is a button that can be added to the bulk actions menu, which is shown when multiple entities are selected in a collection view.                                                                                                                                     |
 | entryPoint                 | (Deprecated) Old name for `backofficeEntryPoint`.                                                                                                                                                                                                                                          |
+| fileUploadPreview       | A file upload preview is a component that can be used to preview files when they are uploaded in the media section. [See the default previews in Umbraco on GitHub](https://github.com/umbraco/Umbraco-CMS/blob/main/src/Umbraco.Web.UI.Client/src/packages/media/media/components/input-upload-field/preview/manifest.ts)                                                                                                                                                                       |
 | globalContext              | A global context is a context instance that is available to all components in the Backoffice. It is used to share state between components and to provide a way to communicate between components. Read more about [Global Context](extending-overview/extension-types/global-context.md). |
 | granularUserPermissions    | A granular user permission is a permission that can be added to a user. It is used to control access to specific parts of the Backoffice.                                                                                                                                                  |
 | headerApp                  | A header app is a component that can be added to the header such as a button or a link. Read more about [Header Apps](extending-overview/extension-types/header-apps.md).                                                                                                                  |