Use preview endpoint for images

[skjnldsv]

Fix#28

[ariselseng]

@skjnldsv
Code looks good. Though In my opinion we should just drop the getMime() in Viewer.vue.
The mime should come from Nextcloud.

It would be very nice if this app could view anything that had had the following conditions: image/** as mimetype and nc:has-previewtrue</nc:has-preview>. That way apps (like mine: https://github.com/cowai/camerarawpreviews) providing previews for other image file types could just work without any integration.

If you look here in my little patch: https://gist.github.com/cowai/1a314c1f32f789ce20b84448cf94be25
You can see that the module 'mime-types' is quite limiting.

EDIT: image/x-dcraw has like 15 different extensions. So it just won't work using that module I think ;)

[skjnldsv]

@cowai Sure, I have a pending branch for that like said in #25 (comment) :)
Let me push it

EDIT: #34

[ariselseng]

Code looks good.