Q: show file names

The filenames with the image numbers do only show in the Info panel for a photo. Select the photo in Photos and enter ⌘I to open the Info panel.

Photos is showing the filename in the Info panel for a photo, but below the thumbnails it will only show titles, not filenames.

If you enable metadata in the View menu in Photos "View > Metadata", then enable all options, you will see the titles.

But you will still have to copy the filenames to the Title field, if you want to see the filenames.

We posted an Apple Script in the User Tip section (Photos for Mac User Tips) to copy the filenames to the title field for selected photos. See:

• Script: Changing the Title to the Filename without Extension
• Script: Batch Changing the Titles to the Filename w/Extension

Alternately, you could get yourself PowerPhotos. It is a great companion to Photos.  In PowerPhotos you can display more metadata below the thumbnails or show your photos in a list view with many different metadata and sort them accordingly.  You can browse your Photos Library in PowerPhotos  with launching Photos and move photos between libraries.  There is a free trial version:  See this page:  Fat Cat Software – PowerPhotos

For example: Browsing several Photos Libraries in PowerPhotos in list view with the filenames showing: