The way that Infuse currently appears to work is that it look for a season/episode number in the filename and if it finds one then it assumes the file is a TV episode and looks it up on thetvdb.com, otherwise it assume it is a film and attempts to look it up on themoviedb.com.
As your files for Planet Earth II episodes do not contains anything that look like a series/episode number it will be attempting to look up a film rather than a TV episode.
I suggest that you rename the episodes to something like:
Planet Earth II S01E01 Islands.mkv
Infuse will see the S01E01 in the filename and therefore assume this is a TV episode rather than a film.