change publicUploadButtonMock to public_upload

Changed jQuery selector for public preview to public_upload to correctly show preview

apps/files/js/files.js

@@ -717,7 +717,7 @@ Files.lazyLoadPreview = function(path, mime, ready, width, height, etag) {
 			console.warn('Files.lazyLoadPreview(): missing etag argument');
 		}
 
-		if ( $('#publicUploadButtonMock').length ) {
+		if ( $('#public_upload').length ) {
 			urlSpec.t = $('#dirToken').val();
 			previewURL = OC.Router.generate('core_ajax_public_preview', urlSpec);
 		} else {