Okay so I just updated to CC 2018 and now photoshop won't let me open mp4 files. I just did this earlier in the day before running the update and had no issues. I'm using windows 7 if that makes any difference.
I'd suggest resetting the Photoshop preferences to default first and check if that helps. You may refer Preferences in Photoshop
Also, which operating system are you using and do you have the latest version of QuickTime installed? Impact of uninstalling Apple QuickTime
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