use svg to generate filetype icon if imagick available
fix new extension
use readImage instead of readImageBlob
check if svg of filetype icon exists
fix doc block of OC\Preview\Provider::getThumbnail()
fix wrong variable name
fix indentation
resize svg without loosing quality
Merge pull request #4949 from owncloud/improve_unknown_preview_backend

use svg to generate filetype icon if imagick available