How Do I View MP4 Video Files On an iPhone or iPad?

Do you have any recommendations on the best way to view MP4 video files on both my iPhone7 or my iPad Air 2?
Whenever someone sends me an email with a MP4 file attached, I can see that there is an attachment to the email, but can find no way of viewing it. For interest, I can view the files on my iMac.
I understand that I will probably need to get or buy an app from the App Store, but would welcome your advice on any specific ones to get.
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BoJo