feat: add widgets schema validation (#3841)

packages/netlify-cms-widget-file/src/index.js

@@ -1,11 +1,13 @@
 import withFileControl from './withFileControl';
 import previewComponent from './FilePreview';
+import schema from './schema';
 
 const controlComponent = withFileControl();
 const Widget = (opts = {}) => ({
   name: 'file',
   controlComponent,
   previewComponent,
+  schema,
   ...opts,
 });
 

packages/netlify-cms-widget-file/src/schema.js

@@ -0,0 +1,5 @@
+export default {
+  properties: {
+    allow_multiple: { type: 'boolean' },
+  },
+};