How do I create a media type with a plus in the type name?

I’m adding an Atom feed, for which the spec says:

An Atom Document, when serialized as XML 1.0, can be identified with
the following media type:
MIME media type name: application
MIME subtype name: atom+xml

However, when I put a plus in the config:

  suffix = "xml"

Sadly Hugo doesn’t like the plus:

ERROR 2018/05/21 17:40:05 media type keys cannot contain any '+' chars. Valid example is "text/css"

Is there some other way to get it to allow the plus in the media type name? I tried adding a string variable, but it was ignored.

You can template your rel="alternate" like this – Set .MediaType.Type to "atom" and .MediaType.Suffix to "xml"… which would be the case anyways.

So define your custom output formats using something like this (I do exactly that):

    suffix = "xml"

  mediaType = "application/atom"
  baseName = "atom" # generated file = <baseName>.<mediaType."application/atom".suffix> = atom.xml
  isPlainText = false

  home = ["HTML", "RSS", "ATOM"] # default = ["HTML", "RSS"]

Ah, Just use the suffix, got it. I’ve written it as:

{{ range .AlternativeOutputFormats -}}
<link rel="{{ .Rel }}" type="{{ .MediaType.Type }}{{ if eq .MediaType.Suffix "xml" }}+xml{{ end }}" href="{{ .Permalink | safeURL }}" title="{{ $.Site.Title }}" />
{{ end }}

Which works just as I need. Thanks!