I’m using UiPath version 2017.1.6435 and the activity Invoke workflow file is being displayed as a simple activity, there is no field to select the .xaml path neither the arguments.
I just uninstalled the UiPath.Core.Activities as tried before, but this time I manually copied the version UiPath.Core.Activities.17.1.6435.21841 of the activity to the UiPath activities path (C:\Users\{yourUser}\AppData\Local\UiPath\Activities).
Then I restarted the UiPath Studio and everything got back to normal again.
It seems that the old version of the activity installed was related to the UiPath 2016 version and this is probably not compatible with the version 2017 (my guess).