Google, until now allowed free photo and videos to be uploaded to Photos services as long as they are compressed. However, the company is changing it now, as of December 6th, videos in unsupported formats are no longer included in the free storage.
In the Google support site, the company added the following note: “videos uploaded after December 6, 2018, take up storage space.” Google Photos supports almost any video format which includes .mpg, .mod, .mmv, .tod, .wmv, .asf, .avi, .divx, .mov, .m4v, .3gp, .3g2, .mp4, .m2t, .m2ts, .mts, and .mkv files. Since it is on the unsupported video page, your files are likely still free.
Rare video formats like VOB or RAW video from a high-end camera will take up storage space going forward. It is also likely that Photos users are tinkering with the file containers to make large, non-video files look like videos. Something of this kind that might have counted as free storage in the past but no more.