Description:
I created an impress presentation with an included video, when I saved it as odp everything went normal, but when I saved it as pptx and reopened it the video was gone.  This happened with mp4, mpeg and wmv
Steps to Reproduce:
1.Create Presentation with video
2.Save Presentation as pptx
3.Close Presentation
4.Open Presentation
Actual Results:
Video is gone
Expected Results:
Video is still there as left
Reproducible: Always
User Profile Reset: No
Additional Info:
I tested it on newest fedora and ubuntu lts version. 
It does not happen when saving to ppt
User-Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0

Created attachment 135632[details]
Test PPTX (created in PowerPoint)
Not only the video is not saved into PPTX, the file produced by LO is not a valid PPTX.  Attaching a valid PPTX created in PP for testing.