File transfer is allowed in meetings even when EnableExternalP2PFileTransfer is set to false in Skype for Business

When the EnableExternalP2PFileTransfer property of the conferencing policy is set to false, you're still able to attach and transfer files in meetings in Microsoft Skype for Business 2016 and Skype for Business 2015 (Lync 2013).

To fix this issue, install one of the following updates:

• For Skype for Business 2016, install the March 6, 2018, update (KB4011725).
• For Skype for Business 2015 (Lync 2013), install the March 6, 2018, update (KB4018290).

Note Lync 2013 was upgraded to Skype for Business in April 2015.

To enable the fix, create In band policy or the GPO:

Inband policy:

$a = New-CsClientPolicyEntry -Name EnableExternalFileTransferInConference -Value "false"
Set-CsClientPolicy -Identity <policy name> -PolicyEntry @{Add=$a}

Group Policy settings:

1. Open the Group Policy Management panel and create a new Group Policy Object.
2. Enter the name of the GPO and leave the source as none.
3. On the Settings tab, right-click on Computer Configuration or User Configuration and select Edit.
4. Right click on registry and select New Registry Item from the dropdown menu. You can deploy the registry key under computer configuration or user configuration.
5. Specify the Registry information in the form as below:
   
   a) Action : "Create"
   
   b) Hive : "HKEY_CURRENT_USER"
   
   c) Key path : HKLM\Software\Policies\Microsoft\Office\16.0\Lync or HKLM\Software\Policies\Microsoft\Office\15.0\Lync
   
   d) Value name : EnableExternalFileTransferInConference 
   
   e) Value type : REG_DWORD 
   
   f) Value data : 0
   
   g) Base: Decimal