SDK throws an exception when link to a header is used

[mlewand]

Provide detailed reproduction steps (if any)

1. Open https://sdk.ckeditor.com/samples/fileupload.html#uploading-dropped-and-pasted-images (note #uploading-dropped-and-pasted-images part).
2. Open the conosle.

Expected result

No exceptions were thrown.

Actual result

Console contains following exception:

Uncaught TypeError: Cannot read property 'length' of undefined

Other details

• Browser: Any
• OS: Any