Have the server send the mime you want. Then, Opera will pop up a save dialog so you can save the file. After you do, in the download status after the download is complete, you can double-left-click the download, to launch the file in the default handler for that file time on the system.

I don't know of a way in Chrome or Opera to edit mime type mappings and edit the actions for them (including the option to open a file type directly (even automatically) when you click a link to it).

There's so more info here if you want to an extension that can change the mime type to something different than what the server sends. But, I don't know if that's what you want or not. Remember that you can download chrome extensions for Opera.

Also, I see there's https://forums.opera.com/topic/1459/where-do-you-set-the-default-actions-for-filetypes-in-opera-20 that pretty much asks the same question as yours.