As you know, today when opening a file shared in Teams (either chat, meeting or Teams) the file opens by default within Teams.
Well, good new, you can now manage this settings and change the default action when opening a file in Teams; you can choose Teams (default), Office apps or web browser
This applies only to Word, Excel or PowerPoint files.
To change this setting, access a location where you have file in Teams (easier is to use the Files app in Teams) and then open the contextual menu of a file to access the Open in option and select Change default
NOTE you will need to have Teams client version 1.4.00.19573 or later