Hi Mathieu Jenkinson,
Thank you for reaching out to us. I'm glad to help you!
First, based on your description and the image you provided, I understand that the issue you are experiencing is when you try to attach an Excel file as a tab in Microsoft Teams, the pop-up modal, instead of showing a list of Excel files for you to choose from, displays a "Documents/General" folder structure, and this issue occurs on both the desktop Teams application and the browser version.
I have attempted to test this on my own Teams environment. I uploaded an Excel file to a channel I created. When I performed the "Add a tab" (+) action and selected "Excel," the system displayed a list of my most recent Excel files (not a folder structure like in your case), and I was then able to successfully add the file as a tab.

To assist you more effectively, could you please help me clarify or provide the following details:
- Could you please confirm if this is an work Teams account right?
- What is your role in the channel where you are trying to add the Excel tab (Owner, Member, Guest)? Have you successfully added an Excel tab in this channel before, or is this the first time you've encountered this situation?
- Is Excel file uploaded directly to the channel’s Files tab?
- Have you tried performing this action in another channel where you are an owner or an active member? This will help determine if the issue is limited to a specific team/channel.
- Are there any other members in the same channel who are experiencing a similar issue?
- What type of channel are you trying to attach the Excel file to (Standard channel, Private channel, Shared channel)?
- Have you tried uploading a new Excel file to that channel, and then tried to add that file as a tab? If so, was the result similar?
- What troubleshooting steps have you already attempted to resolve this issue?
Please understand that our initial response may not always resolve the issue right away. But with your help and a bit more detail, I'll try my best to help you.
Looking forward to your reply!
Sincerely,
Jeanie-H - MSFT | Microsoft Community Support Specialist.