Hardened image file validation by removing custom validation

- Added test to check PHP files cannot be uploaded as an image.

app/Http/Controllers/ImageController.php

@@ -119,7 +119,7 @@ class ImageController extends Controller
     {
         $this->checkPermission('image-create-all');
         $this->validate($request, [
-            'file' => 'is_image'
+            'file' => 'mimes:jpeg,png,gif,bmp,webp,tiff'
         ]);
 
         if (!$this->imageRepo->isValidType($type)) {
@@ -135,7 +135,6 @@ class ImageController extends Controller
             return response($e->getMessage(), 500);
         }
 
-
         return response()->json($image);
     }