Are there any error messages when access Teams files in the channel?
Microsoft 365 admin users have access to diagnostics that can be run within the tenant to verify possible issues with accessing files to Teams. To narrow down this issue, it recommends you try to Run Tests below, which will populate the diagnostic in the Microsoft 365 Admin Center.
https://aka.ms/TeamsAccessFilesInChat
Besides, please check if you have selected the channel 2.0 in Teams client. I did a test and found if you change this mode, user cannot see the file list in Teams channel.
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