@Loa looks like you solved this with AppleScripts. I'm posting an alternative solution here for others that may come across this. I'm the author of the free open source tool,
osxphotos which includes a
batch-edit command that can easily do this. For example, you could run the following:
Bash:
osxphotos batch-edit --title "{original_name} {title}"
which will replace the title of any currently selected photos in Photos to the filename (minus extension) + any existing title. If you wanted the full filename with extension, you'd use
Bash:
osxphotos batch-edit --title "{photo.original_filename} {title}"
and if you wanted to overwrite any existing title instead of appending it, you'd use:
Bash:
osxphotos batch-edit --title "{original_name}"
This tool has a very flexible template system and can also adjust keywords, description (caption), and location.