See https://discuss.gohugo.io/t/custom-output-content-types/4584

Issues that this issue will also solve (will check):

• Upper case tags Misc case issues for URLs #1641
• Pagination links Tag ".net" creates invalid pagination #2110 helpers: fix invalid pagination links #2374 Builtin pagination template doesn´t work with sub root in BaseURL and CanonifyUrls = "false" #1252
• Ugly URL ambiguity Handle uglyURLs ambiguity in Permalink #3102
• Taxonomies vs ugly Taxonomies ignoring uglyurl=true #1989
• Dots Periods generating bad paths #1885
• Permalink for lang roots + ugly Permalink for language-roots wrong when uglyURLs is true #3179
• Quick & useful: Configurable file extension #320 configurable file ext
• Fix base template lookup order for sections #2995 template lookup theme vs project

Other TODOS:

• Rewrite pageLayoutIdentifier to use the descriptor/adapter pattern.
• HandleTODO(bep) output
• Rename OutputType => OutputFormat
• Remove siteWriter.
• Read Output formats from site config
• Read Output format definitions from site config => custom output formats: Read Output formats from site config #3222
• Read Media types from site config => custom output formats: Read Media types from site config #3223
• * Handle types in aliases
• Add Page.OutPutFormats with Permalink, RelPermalink + a way to find AMP etc.
• Base template handling
• Partial handling
• Text vs HTML templates => custom output formats: Handle both text and HTML templates #3221
• * Stats report
• Ref vs relref => custom output formats: Update Rel and RelRef #3224
• Register MIME types in server
• * Handle protocol
• * There is a panic in the themes site build with this branch.
• Check injector + replacements vs suffix