Properly theme folder icons that are accessed though imagePath (#15875)

Properly theme folder icons that are accessed though imagePath

apps/theming/lib/ThemingDefaults.php

@@ -349,6 +349,9 @@ class ThemingDefaults extends \OC_Defaults {
 			} catch (AppPathNotFoundException $e) {}
 			return $this->urlGenerator->linkToRoute('theming.Theming.getManifest') . '?v=' . $cacheBusterValue;
 		}
+		if (strpos($image, 'filetypes/') === 0) {
+			return $this->urlGenerator->linkToRoute('theming.Icon.getThemedIcon', ['app' => $app, 'image' => $image]);
+		}
 		return false;
 	}