We started using mp4 because the game size was to big (over 4Gb).
You can use any format you want, but you also have to change what the game expects.
Because right now the new passages say webm but the files are mp4.
EDIT: partial list of videos you should be mp4 instead of webm in the passages
AA25L1, AA25L3, AA25L4, CBD9, CBD10, CBD15, CBD16, CBD17, CBD18, CBD18_2, CBD22, CBD23, CBD24, CBD24_2, CBD24_3, CBD25, CCA13A, CCA14A, CCA15A, CCB13A, CCB14A, CCB15A, CCC13A, CCC14A, CCC15A, CCD13A, CCD14A, CCD15A