Modify extImgObs so it can use mp4 files.

[GeraldCNelson]

I would like to add an mp4 video to my powerpoint. The documentation says you can add png or emf files. I'd like to ask about modifying external_img so that it would work with code like that below.

library(officer)
my_pres <- read_pptx()
extImgObs <- external_img(src = "graphics/someMP4File.mp4")
my_pres <- add_slide(my_pres, layout = 'Title Only', master = 'Office Theme') %>%
ph_with(value = extImgObs, location = ph_location(left = 2, top = 0) )