Fall back from server generated thumbnail to original image (#10853)

* Add test for falling back from thumbnail to original

* Fall back from server generated thumbnail to original image

src/components/views/messages/MImageBody.tsx

@@ -165,6 +165,14 @@ export default class MImageBody extends React.Component<IBodyProps, IState> {
     };
 
     private onImageError = (): void => {
+        // If the thumbnail failed to load then try again using the contentUrl
+        if (this.state.thumbUrl) {
+            this.setState({
+                thumbUrl: null,
+            });
+            return;
+        }
+
         this.clearBlurhashTimeout();
         this.setState({
             imgError: true,