MP4 attachments

Can anyone tell me how to add an MP4 attachment on a post on the picture gallery? Should I be putting it somewhere else. My file was rejected and as far as I can see MP4 is not a supported file.

Maybe upload to YouTube. Then you can embed it here (don't know how though)

just edit some and upload to youtube. if dont wanna use that you can put video files on photobucket as well. not sure if imgur takes videos.
but if its raw video first edit it some maybe cut something and save it.then upload . if uploading to youtube you can edit it some before uploading there too.
you can make gif animations files from video too , like with this https://giphy.com/create/gifmaker . its online so just upload your video there from computer. then make animation file from part that you want and it will upload it to his own library then you can just share link as picture.

result could be something like this

Correct - MP4 is not supported for upload to albums or attachments. Supported files are bmp doc gif jpe jpeg jpg pdf png psd txt zip. So as the others have mentioned it's usual that videos are stored elsewhere and then linked here.