Unable to open documents in Collabora app

Hello everyone,

I tried to install Collabora Online docker image on my Qnap server (through Container Station) and to configure it to work with my Nextcloud server on a Debian VM.

Now that I’m trying to use it, I can’t open an existing document nor create a new one.

Related log entry:
Warning richdocuments Starting a session failed. Reason: /files/Nouvelle Feuille de Calcul.ods is application/octet-stream and is not supported by RichDocuments app.

Why can xlsx/ods files not be opened?
Do you have any idea to help me?
Thank you in advance.