The extension is .XML but indeed, the content is in OPML format. Have a look at it, in any kind of text editor. If the platform you are using only accepts .opml extension, simply "rename" it as you would like to, it should work.

New feature requests are welcome, I will try to do my best to implement them.

Regarding Google's strategy: Yes, it's a shame and upsetting, that's why I take my time and the effort to create this :)

Thank you for your rating and opinion!

UPDATE: Okay, so I installed Thunderbird to figure this stuff out and managed to import both as XML and with the extension rename as an OPML file. Sorry, but the problem is on your side! I went into "Manage feed subscriptions", "Import" and Thunderbird successfully imported it, in both ways.

(You can choose upon import from a dropdown, which file type you are looking for (XML or OPML), as usual in file browsing window.)

It created a "YouTube - subscriptions" folder, and immediately fetched the channel feeds.