This function is also used in a way such as:

```
	$preview = new \OC\Preview(\OC_User::getUser(), 'files');
	$info = \OC\Files\Filesystem::getFileInfo($file);
	if (!$always and !$preview->isAvailable($info)) {
		\OC_Response::setStatus(404);
	} else {
		$preview->setFile($file);
		$preview->setMaxX($maxX);
		$preview->setMaxY($maxY);
		$preview->setScalingUp($scalingUp);
		$preview->setKeepAspect($keepAspect);
	}
```

Which won't work anymore since `setFile` is used instead of passing the file in the constructor. Fixes a regression in master.

Ref https://github.com/owncloud/core/issues/11078

Ensures that Basic Auth works properly for APIs and removes the need for some even uglier lines of code.

Resolves https://github.com/owncloud/core/issues/10991 failure 4

Fixes https://github.com/owncloud/core/issues/11133

At the previous point not all apps were initialized. Now the basic auth check happens together at the same location as all others.

Fixes https://github.com/owncloud/core/issues/11129

We do not support updates from 6 to 8 directly, therefore we can remove those upgrade routines.

Thumbnail generation

Removed Log

Added requested changes

Added requested changes.

- Fix code style
- Add exception if file does not exist
- Switch route styling

Replaces https://github.com/owncloud/core/pull/10805

Fix codestyle

Fix codestyle

Migrate to appframework

Fix typo

Some code that I also used for https://github.com/owncloud/administration/pull/11

if dbuser exists try a different one, owncloud/core#108

changed outdated comment

corrected length calculation

changed indentation for else clauses

You can't add int here as hint...

Conflicts:
	core/templates/layout.guest.php

Step one, open guest layout for different styles.