In a Microsoft Skype for Business Server 2015 environment, you create a file transfer filter configuration for the Edge Server to block a user's ability to transfer certain type files by using a Skype for Business Server client in an external network. When a Skype for Business Server client that's enabled by Microsoft Unified Communications Web API (UCWA), such as a Skype for Business on Mac client, is externally talking to an internal client, you may notice that it doesn't honor the file transfer policy as configured.
This issue occurs because the external file transfer's traffic doesn't traverse the Edge Server. Therefore, the external file transfer is enabled.
To fix this issue, install the January 2019 cumulative update 6.0.9319.537 for Skype for Business Server 2015, Core Components.
Note After you apply this update, you can run the following cmdlet to prevent external network users including the UCWA users from being able to transfer files to internal users:
New-CsConferencingPolicy -Identity BlockExternalP2PFileTransfer - EnableP2PFileTransferOutsideIntranet $False
This example creates a new policy to block an external file transfer. Then you can use Grant-CsConferencingPolicy to assign it to a user account.
After you apply this update, the Skype for Business Server will give you a way to prevent the Skype for Business on Mac user from being able to transfer files externally to an internal user.