You Can’t Modify this. it is widget code which handles all error and propagates to UI.
In File Dropper Widget the File Verify Action will not return actually error.
You can file an issue on the GitHub repository regarding this use case.
You can see in below code it is hardcode warning message.
https://github.com/JelteMX/mendix-file-dropper/blob/3829b144df09e4a9f061456f925202dcf2398e7b/src/store/fileDropperStore.ts#L63