I read this question but it doesn't fully answer mine.
In the linked question, the OP asks what he must do to have his custom media type registered. I do not need that.
All I'm asking is, that if I am correct in assuming that, technically, all I have to do to create a custom media type is to, well.. just use it in my communication?
Basically, what I want to do is that my website offers a download of a file, described as
application/x-my-custom-thing. On the client, the user has files ending with
.my-custom-thing set as to open with the application I ship along.
So, what should happen is that, when the user clicks on the link, my application opens and reads the file and does stuff with it.
Are media types even the proper use case for this? Or, should I maybe "create" my own protocol in the sense of
mycustomthing://example.com/the-file-to-use? Will that work with every (reasonably modern) browser?