Rename pictures in Photos

There are two possibilities:

1. You use the Share mechanism. That gets you the preview of the photo. It's a good enough for most things version but it has none (or very little) of the metadata. So you get an email with the filename not the title.
2. You export: File -> Export and you have the option to export a jpeg and you can opt to use the title as filename on the exported file. Attach that to the email and you get what I suspect you want.

In Photos you add a title, which you do just above the filename you have indicated here. There is no way to rename a file in Photos except when exporting.