EasyMDE: The server did not return a valid json.

I got this message after uploading an image:

````EasyMDE: The server did not return a valid json.````

My code is the following:
```js
new EasyMDE({
element: document.getElementById('EasyMDE_field'),
hideIcons: ['image'],
showIcons: ['upload-image'],
imageUploadEndpoint: "https://domain.de/CMS/functions/img_upload.php"
});
```
and in the img_upload.php:
```php
if(isset($_FILES['image'])) {
$uploaddir = '../data/';
$uploadfile = $uploaddir . basename($_FILES['image']['name']);

if(!is_uploaded_file($_FILES['image']['tmp_name'])) {
echo json_encode(array('error'=>'importError'));
echo "can't upload";
}
if (move_uploaded_file($_FILES['image']['tmp_name'], $uploadfile)) {
echo json_encode(array('data'=> array('filePath'=>$upload_json)));
}
}
```
The images are getting uploaded, but there will be no link insert in the Textfield, instead i got an error message
and the json code loos so:
````json
{"data":{"filePath":"..\/data\/image.png"}}
````

Start with EasyMDE's image-upload handling and the shown img_upload.php endpoint, comparing the response produced by the upload path with the JSON shape returned in the report. Reproduce an image upload, check the complete server response, and consider the work done when the editor accepts it and inserts the image link.

Written by the indexing model from the issue text.