The domain external.planio.media is operated by Planio and runs on the same secure server platform as your Planio account, so absolutely no need to worry. We use this domain to embed external media content in wiki pages, issue descriptions, chat messages and so on.
As you might know, Planio will automatically expand and embed links to certain online media such as Twitter messages, YouTube videos, and images when you copy and paste them anywhere within Planio. In order to achieve this, your browser needs to load content from these external services and sometimes execute external code, such as JavaScript. To do this in a secure way and to avoid that the external services get access to what's currently shown in your browser window, we're embedding all content within an isolated section called IFrame. The best way to enforce this isolation is to load content from a different domain.
So what actually happens is that all external media content is embedded on external.planio.media which in turn is embedded into your Planio account. That sounds overly complicated but it's all happening in the background and you don't need to do anything. Furthermore, it adds a layer of protection that makes sure your data stays where it belongs. This allows content from external services such as Facebook or Google to be displayed but it prevents any third parties from finding out about your Planio data.