MP4 is a good place to start.
For broad browser support, use MP4 with H.264 video and AAC audio. The filename alone cannot tell us which codecs a video uses.
WebM depends on your browser.
VP8 / VP9 video with Opus or Vorbis audio plays in many modern browsers. Device support varies.
MKV is not reliably browser compatible.
MKV, MOV, HEVC / H.265 and some audio formats may fail to play or have no sound. Ask your host for an MP4 copy. Trashcanmedia plays the original file and does not transcode it.
Slow start or buffering?
Try a lower bitrate copy, check the host's upload speed, or pause for a moment. Seeking is supported when the file and browser support it.