[fix] mime-types v3 HAS BUG

[chunlampang]

Describe the bug

koa v3 replaced cache-content-type with mime-types
but mime-types current version has a bug jshttp/mime-types#138

Actual behavior

ctx.type = 'mp4' will set application/mp4 to the content-type

Expected behavior

ctx.type = 'mp4' will set video/mp4 to the content-type

Should be rollback this change #1886 or use the mime-types v2 instead of v3

Checklist

• I have searched through GitHub issues for similar issues.
• I have completely read through the README and documentation.
• I have tested my code with the latest version of Node.js and this package and confirmed it is still not working.