Add webp support

[jokakilla]

Requesting support to show webp images in the notifications.
Currently it's not shown embeded in the notification.

I've seen there is a mime type check for jpg/png in a util class. I already added "image/webp" on a local build and all is working fine. I'll prepare a PR asap.

[jokakilla]

Would be awesome if the PR could be reviewed/merge soon.
Currently the functionality is broken in my usecase and in my scenario I don't have the option to switch back from webp to jpg/png and doing an conversion from webp to jpg is also not an option.

[ki9us]

webp images work fine in the web app

In the android app, I can only get jpegs to preview: