fix: fix the way of verifying the response file disponibility

src/index.ts

@@ -370,7 +370,7 @@ export default class ImageTool implements BlockTool {
    * @param response - uploading server response
    */
   private onUpload(response: UploadResponseFormat): void {
-    if (response.success && response.file.url) {
+    if (response.success && Boolean(response.file)) {
       this.image = response.file;
     } else {
       this.uploadingFailed('incorrect response: ' + JSON.stringify(response));