make the mime type checker for flow available to regular users

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

apps/workflowengine/lib/Check/FileMimeType.php

@@ -154,4 +154,8 @@ class FileMimeType extends AbstractStringCheck implements IFileCheck {
 	public function supportedEntities(): array {
 		return [ File::class ];
 	}
+
+	public function isAvailableForScope(int $scope): bool {
+		return true;
+	}
 }