CachedNetworkImage may cache some non-image content , and can't work well for this image url again

## 🐛 Bug Report

In my situation , I use some backend service to get some images. But if this backend service is not working well , it may return a text content like this:
```


Fatal error: Call to a member function isInCOS() on a non-object in /data/www/openkm/app/controllers/PagesController.php on line 174

```

But I found that the CachedNetworkImage will use this text ,and store it as a html file in the cache path .

And next time when request for the image again, this html file is used for the cache , and will make the image codec exception.(error with "Invalid image data")

So this image can never be get again!!

I think maybe a checking for content before cache is needed?

**Version:** 3.2.3

**Platform:**
- [x] :iphone: iOS
- [x] :robot: Android