A common reason a user cannot attach a file in an external room is that the user doesn't have the entitlement Can share files externally enabled.
Check if the user has this entitlement by following these steps:
2. Select Begin Session.
3. Navigate to Accounts > Browse Accounts and select the user whose entitlement you want to check.
4. In Entitlements, check if the entitlement Can share files externally is enabled (Figure 1):
Figure 1 Can share files externally
5. If it is set to No, change this entitlement to Yes.
6. Select Save.
7. Return to Symphony and attempt to attach a file in an external room. This now should succeed (Figure 2):
Figure 2 Attach button