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@AoW Aha, that makes sense. You can create a custom theme for your blog and add the file that way. It's not too intuitive, but once you edit a custom theme, you can create a new file by pressing New Template.
When asked to provide a folder and file name, enter content/podcast-feed.xml (you can name it whatever you want, but the content/ prefix should be there). Then paste the content of your XML file into the text area and hit Update Template. The feed will be available at https://your.podcast.example.com/podcast-feed.xml.