refactor: use static method for supported MIME types

Signed-off-by: ernolf <raphael.gradenwitz@googlemail.com>

lib/private/Preview/ImaginaryPDF.php

@@ -8,7 +8,7 @@
 namespace OC\Preview;
 
 class ImaginaryPDF extends Imaginary {
-	public function getMimeType(): string {
+	public static function supportedMimeTypes(): string {
 		return '/application\/pdf/';
 	}
 }