A common reason a user cannot attach a file in an external room is that the user doesn't have the Can share files externally entitlement enabled.
Check if the user has this entitlement by following these steps:
1. In Symphony, click Settings in the upper right-hand corner, then click General (Figure 1):
Figure 1 Settings/General
2. Click Go to AC Portal at the bottom of the screen (Figure 2):
5. Click Save
6. Return to Symphony and attempt to attach a file in an external room. This now should succeed (Figure 4):
Figure 4 Attach Button