Emacs users sooner or later disable all the GUI stuff, and usually also get rid of system alerts and file pickers.

But if you just once want to pick a file using your operating system’s file picker, these three temporary variable overrides will do:

(let ((last-nonmenu-event nil)
      (use-dialog-box t)
      (use-file-dialog t))
  (call-interactively #'find-file))

If you’re in this situation, you likely already know use-dialog-box and use-file-dialog because you’ve set both to nil globally.

The interesting piece here is last-nonmenu-event: this tricks the interactive call of find-file into believing you used the mouse – which apparently is a prerequisite to make the system dialog appear, even though the docs seem to say that’s not always the case (emphasis mine):

use-file-dialog is a variable defined in ‘C source code’.

Its value is nil

Non-nil means mouse commands use a file dialog to ask for files. This applies to commands from menus and tool bar buttons even when they are initiated from the keyboard. If ‘use-dialog-box’ is nil, that disables the use of a file dialog, regardless of the value of this variable.

Now “Open File” in the main menu is bound to find-file, so the emphasized sentence doesn’t hold true.

But the last-nonmenu-event trick works. This variable’s docs say:

Last input event in a command, except for mouse menu events. Mouse menus give back keys that don’t look like mouse events; this variable holds the actual mouse event that led to the menu, so that you can determine whether the command was run by mouse or not.

So there you go – put the above code snippet into a function and bind it to another key and you will have a quick way to open files with your system file picker.