"open with" default app not working Mac OS 10.14.3 [Mojave]:

QuickTime Player is set as the default app for Finder's "open with" for .mp4 files on my Mac.  This is what I want.  But when I double-click an mp4 file, the kid3 app [a tag editor] launches instead of Quicktime Player!? 

Why and how is kid3 invisibly hijacking the default app for "open with" for mp4 files? I found nothing related to this in kid3's preferences.
I sadly cannot explain this phenomenon. However, here is a hacky workaround that should resolve the issue:


Right click the app you do not want to open mp4 files, and select Show Package Contents.
Navigate to the Contents folder and locate the Info.plist file. Make a backup copy of this file, then open the original in TextEdit (or a similar program).†
Locate the section which resembles the below, and delete it. Save and close the file, and reboot your computer with "Reopen windows when logging back in" disabled.‡


&lt;key&gt;CFBundleTypeExtensions&lt;/key&gt;
&lt;array&gt;
    &lt;string&gt;mp4&lt;/string&gt;
&lt;/array&gt;


Replace mp4 with whatever file extension you don't want the program to open.

I've never used kid3, but this successfully prevented Fission from opening mp3 files by default, as opposed to QuickTime.

(I think you might need to bypass Gatekeeper the next time you open the app? Not sure.)



† If the file looks like gobblygook: close TextEdit, open the Terminal, type "plutil -convert xml1 " (without the quotes, but with a space at the end), drag Info.plist into the terminal window, and press enter. Info.plist should now appear correctly in TextEdit.

‡ A reboot is not strictly necessary, but serves as a quick and definite way to force macOS to recognize the change. If you don't reboot, you will have to wait some (undeterminable) amount of time for your change to take effect.