Enhancement: Simplify mime type checking

We've removed the dependency to GuzzleHttp from our oc10 app package. It was used for mime type checking only. Instead we now rely on a mime type checker that is already bundled with oc10 core.
IMPORTANT: this enhancement is needed to reach compatibility with oc10.11 and maintain backwards compatibility with oc prior to oc10.11. This would not be easily doable when still relying on GuzzleHttp because its major version was updated from 5 to 7 in oc10.11.

https://github.com/owncloud/web/pull/7605
https://github.com/owncloud/web/pull/5933
