[6.x] Fix asset selection in Markdown fieldtype (#14715)

Co-authored-by: Jason Varga <jason@pixelfear.com>

Author: duncanmcclean

src/Http/Resources/CP/Assets/AssetsFieldtypeAsset.php

@@ -15,6 +15,7 @@ public function toArray($request)
         $data = [
             'id' => $this->id(),
             'basename' => $this->basename(),
+            'reference' => $this->reference(),
             'extension' => $this->extension(),
             'size' => Str::fileSizeForHumans($this->size()),
             'isImage' => $this->isImage(),

tests/Http/Resources/CP/Assets/AssetsFieldtypeAssetTest.php

@@ -39,6 +39,7 @@ public function it_returns_expected_data()
 
         $this->assertEquals('test::img/photo.jpg', $resource['id']);
         $this->assertEquals('photo.jpg', $resource['basename']);
+        $this->assertEquals('asset::test::img/photo.jpg', $resource['reference']);
         $this->assertEquals('jpg', $resource['extension']);
         $this->assertEquals('/assets/img/photo.jpg', $resource['url']);
         $this->assertArrayHasKey('downloadUrl', $resource);