Hi @YU Lucy - DXC
To narrow down this issue, I have some questions to confirm with you:
1)Does anyone else have the same issue in your organization?
2)Have you tried to login Teams web client to see if it can be reproduced?
Microsoft 365 admin users have access to diagnostics that can be run within the tenant to verify possible issues that affect uploading files to Teams chat. Select Run Tests, as follows. This will populate the diagnostic in the Microsoft 365 Admin Center.
https://aka.ms/TeamsUploadFilesInChat
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