filesystem: allow @ in file names

It's apparently common in files from Google calendar.

The whole regex probably needs to be reconsidered, but for now just make
this work and keep it simple.

storage/filesystem.go

@@ -29,7 +29,7 @@ type filesystemBackend struct {
 }
 
 var (
-	validFilenameRegex  = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9_-]+(.[a-zA-Z]+)?$`)
+	validFilenameRegex  = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9@_-]+(.[a-zA-Z]+)?$`)
 	defaultResourceName = "default"
 )