Interaction buttons - Unexpected actions

I set up a slide on Captivate 9 with 2 buttons on it. The first is set to open an external file, the second is set to pause the slide and return the user to a previous slide when selected. When tested the slide pauses as expected. If the second button is selected the project moves back to the expected slide. However if the first button is selected it opens the external file as expected but once the user has closed the external file the Captivate project continues automatically to move on to the next slide and does not pause. How do I fix this?