What is the Mime type to display notifications?

I want to replace the ubuntu notification box. What I mean by “notification box” is the little popup that shows up when I change my music on spotify, for example. From what I can tell the OS receives a message with the information and uses Mimeinfo.list to know which program to call and display that message.

Now, I already did some changes with this in the past. Like pointing

image/png=/myCustomImageProgram

I’m looking at the Mimeinfo.list file to try and point the message types from the default to my current program like in the example above, but I have absolutely no idea what type refers to the notifications. Anyone knows what is it?