**editor config**：
```javascript
ClassicEditor
.create( editorElement, {
ckfinder: {
uploadUrl: '/ckfinder/upload.php'
}
} )
.then( ... )
.catch( ... );
```
`uploadUrl` response format:
```json
{
"uploaded ": true,
"error": {
"message": "no error"
},
"url": "https://example...