I need you to define App-ID for transfer file in skype. In yahoo-im App, I see the yahoo-file-transfer App-ID, but not in Skype.
I think is gonna be difficult, skype uses a proprietary implementation and doesn't follow any RFC so first of all the PAN firewall needs to decrypt the sesion [if the skype client uses certificate pinning you're almost screwed] and after that need to reverse engineering the protocol.
Found a bit old topic but if anyone should still need to block Skype file transfers then do google search "skype admin guide".
You can't decrypt Skype traffic so you have to use Group Policy to limit what Skype is permitted to do.
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