remove mime type from metadata; include extension

[feross]

this is to remove the mime-component dependency which adds 35kb to the bundle size.

this should bump the major version since it’s a breaking change. it’s pretty easy to migrate to the new version by simply requiring mime-component and calling mime.lookup(file.extension).

closes #7

[DamonOehlman]

Thanks @feross - I'll merge that now and push a new version (bump major as suggested) :)

[feross]

Thanks for the fast merge and publish!

[DamonOehlman]

No worries - all done :)