Handle errors from imagecreatefrom* functions where the extension does n...

...ot match the file mimeType

I've attempted to upload a JPG with a PNG extension using the Uploader CakePHP plugin and the the following error message:

```
imagecreatefrompng(): 'xxx.png' is not a valid PNG file [APP/Vendor/mjohnson/transit/src/Transit/Transformer/Image/AbstractImageTransformer.php, line 55]
```

This is my attempt at handling that.