I have tried to insert a video-phragment into an Impress-presentation, but the program refuses to recognize any video-file format I have tried so far. The file was downloaded in MP4 format, I tried converting using Prism video-convertor from NCH Software to MOV, MPG, AVI, WMV… all without any success. I use the latest Libre-Office version I could get: version 5.2.5.1 (x64).
I have the same problem 
I also have this problem. When I select an mp4 file from the “Insert Audio or Video” entry in the View pull down menu or from the movie icon in the slide master, I can select the file, but it always responds “The format of the slected file is not supported.”
If I open the file using Windows Explorer it plays correctly