<iframe width="560" height="315" src="https://www.youtube.com/embed/RwN8Sz5I_bA" frameborder="0" gesture="media" allow="encrypted-media" allowfullscreen></iframe>

This script works only to check for the attachments that are already present for the current issue. If you need to check for attachments that are being added in the transition screen, please use !hasInput("attachments") instead of size(attachments) == 0.

See also