In choose application, command line arguments no longer possible because app must be selected. previously it was possible to define mpv --pause as default action, now not, but previous definitions still work.
If I recall correctly, the choose application menu is being rolled back to the original version due to the missing options as you've mentioned. I don't know exactly when this will happen but it should be fairly soon.
*** This bug has been marked as a duplicate of bug 460985 ***