Phots is only showing the titles of the photo below the thumbnail. iPhoto used the filename as the default title, if no title had been assigned, but Photos does not do that. If you want to see the filenames below the thumbnails, you have to copy the filename to the title field.
I wrote script, that can can copy the filenames to the titles as a batch for all selected photos, see:
More batch changing scripts are here: Thematic Index to Photos for Mac Related User Tips:
OT published many of these scrips as compiled versions, so you can run them from the Dock: www.oldtoadstutorials.net/No.P01.html
I like to use PowerPhotos as a second browser for my Photos Library. It can show the Photos Library in a list with metadata displayed in columns or as a grid with titles and subtitles. You can set the subtitle to the filename, the date, or more. These options are available in the free trial version of Power Photos.
If you want to use it to find duplicates or to merge libraries, you need the full, paid version however.
As an added bonus, you can make the background darker.