Insufficient Privileges Error When Downloading Files or Attachments While Logged In as Another Salesforce User

This security restriction is intentional and applies globally to all user impersonation scenarios in Salesforce. It cannot be bypassed or configured away, and this behavior is by design.
Who is affected?
This restriction applies to all of the following user types when logged in as another user:

• Salesforce Support agents
• Salesforce Partners
• System Admins and users with Modify All Data access in your organization

What happens?
When any of these users attempt to download files or content while logged in as another user, they receive the following message:

Insufficient Privileges You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator if access is necessary. For more information, see Insufficient Privileges Errors.

Why does this restriction exist?
This security measure ensures that sensitive files cannot be downloaded or exfiltrated through impersonation sessions, protecting both the organization's data and user privacy.