Output filename in views I have an uploaded file my_cat.jpg
Upon upload, the file-entity's name is by default populated with the file's filename.

Try one: In views, adding a field file:name to my view first looks like the correct answer.
But that's the file-entity's name, which can, even after the upload, be changed to eg. My cat, while the filename on the disk will never change.

Try two: If i add a file:path field, i can decide if it should output the Download-URL or the internal Filestream-Adress.
But no way to leave out the path completely... 
From the field's name, that's not a big surprise.

So, how can i get just the filesystem-filename my_cat.jpg?
In Drupal 8: in Relationships, add a relationship to your image (mine looks like: Image, (field_image:target_id), Content, Appears in: image). In the next step select "Require this relationship". Now, in Fields, you have the filename field available.