It is not only for urgent matters. You can see an option which is to replace or to amend documents. Just try uploading, it won’t hurt your application.
It didn’t say that it is exclusive for IRCC applications. As long as you have UCI and Application Number, you can raise webform.
Use this link
https://secure.cic.gc.ca/enquiries-renseignements/canada-case-cas-eng.aspx?AspxAutoDetectCookieSupport=1