Clients are able to see files that are uploaded to tasks in a task list that is marked as Visible to Client. Clients are not able to see files that are uploaded directly to the File page.
Only administrators and team members can view files uploaded on the file page.
Here are the steps to upload a file that your client can see:
1. Have a task list visible to clients. See How do I share task lists & tasks with clients.
2. Create a task related to the file that you want to upload.
3. Upload the file as part of a comment.
When your client logs in, they will see your uploaded file in their file page, or attached to the task that you uploaded it to.