Suggestion: The handleChange would be useful as a prop for the contentExplorer

https://github.com/box/box-ui-elements/blob/3ef104035f92441ba67c396641b3ab859c038bdd/src/elements/content-uploader/UploadInput.js#L10

I am working with Box Api using the scaffolding and building the appliction from different elements.
My suggestion would be to add an event that'd allow developers to control the selection of contentUploader via props passed down or at least be able to hook onto an event that's exposed at the top level.

I went through the docs and was really surprised not to see a simple code to work;

```ts
this.currentContentExplorer = new Box.ContentExplorer();
this.currentContentExplorer.on('chooseFiles', (files: Entry[]) => {
if(customValidation(files)) this.attachmentsService.upload(files);
});
```

OR

```ts
this.currentContentExplorer = new Box.ContentExplorer();
this.contentExplorerProps = {
contentUploaderProps: {
onSelection: (selectedFiles: Entries[]) => boolean; // tells if the files selected are valid to be displayed in the list
}
}
this.currentContentExplorer.show(
this.getCurrentFolderId(),
this.ssoService.refreshedToken),
this.contentExplorerProps
);
```