How do you want to open this file?

• Run / gpedit.msc (edit group policy)
• locate and expand Computer Configuration / Administrative Templates / Windows Components now click on File Explorer
• on the right panel, double click on "Do not show the 'new application installed' notification" and set it to "Enabled"

This policy removes the end-user notification for new application associations. These associations are based on file types (e.g. *.txt) or protocols (e.g. http:)

If this group policy is enabled, no notifications will be shown. If the group policy is not configured or disabled, notifications will be shown to the end user if a new application has been installed that can handle the file type or protocol association that was invoked.

I never found a solution - maybe it's even intended by MS to push their own apps.

The following workaround however solves it pretty well without interrupting my workflow.

Install AutoHotkey and add this to the top of your script:

; auto close annoying windows
Loop {
  Sleep 500
  if (WinActive("ahk_class Shell_Flyout")) {
    WinGetPos , , , cw, ch
    ; if (cw=710) {
      ; How do you want to open this file? / Keep using this app
      Send {Enter}
    ; }
  }
}

You may need to enable the size check if Shell_Flyout is used for anything else.