Page Media single file "View" action doesn't work for most files

[robertbak]

The button works for image files (since they have "extras" filled), but doesn't work for most other file types (null/undefined instead of the filename is inserted into the url).

This was introduced in dbdd6e2 and as far as I can tell there's no reason for the change.

Steps to reproduce:

1. Drop an mp4 or pdf file into page media
2. Save
3. Try using the view button

Result:
Page url with undefined instead of the file name is opened

Expected:
Open the file in a new tab