In the March 2018 update for Lync2013/SfB2015 and SfB2016 a new setting was included that can disable file upload to external conferences (Federated and Anonymous).
Before this update the Lync/SfB Client didn't had a setting that could prevent file upload to external meetings:
We can configure the EnableExternalFileTransferInConference setting with Inband Policy or Register Key:
Inband Policy
$a = New-CsClientPolicyEntry –Name “EnableExternalFileTransferInConference” –Value $false
Set-CsClientPolicy –Identity <Policy Name> –PolicyEntry @{Add=$a}
Command Prompt
Lync 2013/Skype for Business 2015
reg add HKLMSoftwarePoliciesMicrosoftOffice15.0Lync /v EnableExternalFileTransferInConference /t REG_DWORD /d 0 /f
Skype for Business 2016
reg add HKLMSoftwarePoliciesMicrosoftOffice16.0Lync /v EnableExternalFileTransferInConference /t REG_DWORD /d 0 /f
PowerShell
Lync 2013/Skype for Business 2015
New-Item -Path "HKLM:SoftwarePoliciesMicrosoftOffice15.0Lync" -Force
New-ItemProperty -Path "HKLM:SoftwarePoliciesMicrosoftOffice15.0Lync" -Name EnableExternalFileTransferInConference -Type DWORD -Value 0 -Force
Skype for Business 2016
New-Item -Path "HKLM:SoftwarePoliciesMicrosoftOffice16.0Lync" -Force
New-ItemProperty -Path "HKLM:SoftwarePoliciesMicrosoftOffice16.0Lync" -Name EnableExternalFileTransferInConference -Type DWORD -Value 0 -Force
Please note that this registry key is available on a User Level (HKCU) and Local Machine (HKLM).
After disabling the upload the users won't be able to upload file to External Conferences:
Please note that EnableExternalFileTransferInConference won't block users to download files from external conferences:
The EnableExternalFileTransferInConference setting is available in the following Lync/SfB Client versions:
Office 365 ProPlus/Office Professional Plus 2016 Click-to-Run (after 16.0.8827.2082)
https://technet.microsoft.com/en-us/library/mt592918.aspx
March 6, 2018, update for Skype for Business 2015 (Lync 2013) (KB4018290) (15.0.5015.1000)
https://support.microsoft.com/kb/4018290
March 6, 2018, update for Skype for Business 2016 (KB4011725) (16.0.4666.1000)
https://support.microsoft.com/kb/4011725