utilize oc10 preview providers for allowed mime types in preview app

[kulmann]

ownCloud 10 lets admins define preview providers in the config.php file, see e.g. https://github.com/owncloud/files_mediaviewer#supporting-more-media-types for details

For the preview app we should utilize this for automatically registering additional supported file types.

When we introduce configuration management for web, we could inject preview provider settings into the config.json endpoint of the web-oc10-integration app.

[kulmann]

Since I've heard frustration about the complexity of the proposed solution: a simple first step to achieve the same is to allow configuring the preview providers in the config.json of ownCloud Web. I.e. configure it in config.php server side and in config.json client side.

[mmattel]

Imho it is important to differenciate based on experience, if I recall properly:

1. The preview provider is able to create thumbnails for multimedia formats which are then shown in the web ui.
   This is easy because you can add items in config.php if available and enable thumbnail rendering if necessary in the imagick config. This is all documented.
2. When the file gets clicked and the browser can disply the format, the file gets previewed by the browser but gets downloaded if the browser is not capable.
   This implies that for each format to be previewed, the frontend needs to be capable to do so. This is documented too.

The question is, which media formats need to be added to be shown, satisfying which topic above.