I found an archived answer this problem but its been archived.
When using RDP via an RDG to an RDSI can launch and hear attachments from Outlook by double clicking the .mov icon but there is no video.
I installed VLC on the end users PC, RDG, and RDS and set as the default on all 3.
The default player on the RDS/RDG server in Outlook is still defaulting to Windows media player.
The way outlook open is on the landing page they log into, if he RDP's straight into the server it is fine, just the link for Outlook only doesnt work.
The previous solution that worked for someone else was
Please download RemoteApp .rdp file from RD Web Access page via third party browser, then open the .rdp file with Notapad.exe and add a line "videoplaybackmode:i:0“, after that connect via the modified .rdp file to see whether it works.
If it works, please add the property via PowerShell cmdlet below on RD Connection Broker:
Set-RDSessionCollectionConfiguration -CollectionName "collectionname" -CustomRdpProperty "videoplaybackmode:i:0"