Add ContentTypes to config

This is an empty struct for now, but we will most likely expand on that.

```
[contentTypes]
  [contentTypes.'text/markdown']
```

The above means that only Markdown will be considered a content type. E.g. HTML will be treated as plain text.

Fixes #12274

media/config.go

@@ -71,11 +71,15 @@ func init() {
 		EmacsOrgMode:     Builtin.EmacsOrgModeType,
 	}
 
-	DefaultContentTypes.init()
+	DefaultContentTypes.init(nil)
 }
 
 var DefaultContentTypes ContentTypes
 
+type ContentTypeConfig struct {
+	// Empty for now.
+}
+
 // ContentTypes holds the media types that are considered content in Hugo.
 type ContentTypes struct {
 	HTML             Type
@@ -85,13 +89,36 @@ type ContentTypes struct {
 	ReStructuredText Type
 	EmacsOrgMode     Type
 
+	types Types
+
 	// Created in init().
-	types        Types
 	extensionSet map[string]bool
 }
 
-func (t *ContentTypes) init() {
-	t.types = Types{t.HTML, t.Markdown, t.AsciiDoc, t.Pandoc, t.ReStructuredText, t.EmacsOrgMode}
+func (t *ContentTypes) init(types Types) {
+	sort.Slice(t.types, func(i, j int) bool {
+		return t.types[i].Type < t.types[j].Type
+	})
+
+	if tt, ok := types.GetByType(t.HTML.Type); ok {
+		t.HTML = tt
+	}
+	if tt, ok := types.GetByType(t.Markdown.Type); ok {
+		t.Markdown = tt
+	}
+	if tt, ok := types.GetByType(t.AsciiDoc.Type); ok {
+		t.AsciiDoc = tt
+	}
+	if tt, ok := types.GetByType(t.Pandoc.Type); ok {
+		t.Pandoc = tt
+	}
+	if tt, ok := types.GetByType(t.ReStructuredText.Type); ok {
+		t.ReStructuredText = tt
+	}
+	if tt, ok := types.GetByType(t.EmacsOrgMode.Type); ok {
+		t.EmacsOrgMode = tt
+	}
+
 	t.extensionSet = make(map[string]bool)
 	for _, mt := range t.types {
 		for _, suffix := range mt.Suffixes() {
@@ -135,32 +162,6 @@ func (t ContentTypes) Types() Types {
 	return t.types
 }
 
-// FromTypes creates a new ContentTypes updated with the values from the given Types.
-func (t ContentTypes) FromTypes(types Types) ContentTypes {
-	if tt, ok := types.GetByType(t.HTML.Type); ok {
-		t.HTML = tt
-	}
-	if tt, ok := types.GetByType(t.Markdown.Type); ok {
-		t.Markdown = tt
-	}
-	if tt, ok := types.GetByType(t.AsciiDoc.Type); ok {
-		t.AsciiDoc = tt
-	}
-	if tt, ok := types.GetByType(t.Pandoc.Type); ok {
-		t.Pandoc = tt
-	}
-	if tt, ok := types.GetByType(t.ReStructuredText.Type); ok {
-		t.ReStructuredText = tt
-	}
-	if tt, ok := types.GetByType(t.EmacsOrgMode.Type); ok {
-		t.EmacsOrgMode = tt
-	}
-
-	t.init()
-
-	return t
-}
-
 // Hold the configuration for a given media type.
 type MediaTypeConfig struct {
 	// The file suffixes used for this media type.
@@ -169,6 +170,58 @@ type MediaTypeConfig struct {
 	Delimiter string
 }
 
+var defaultContentTypesConfig = map[string]ContentTypeConfig{
+	Builtin.HTMLType.Type:             {},
+	Builtin.MarkdownType.Type:         {},
+	Builtin.AsciiDocType.Type:         {},
+	Builtin.PandocType.Type:           {},
+	Builtin.ReStructuredTextType.Type: {},
+	Builtin.EmacsOrgModeType.Type:     {},
+}
+
+// DecodeContentTypes decodes the given map of content types.
+func DecodeContentTypes(in map[string]any, types Types) (*config.ConfigNamespace[map[string]ContentTypeConfig, ContentTypes], error) {
+	buildConfig := func(v any) (ContentTypes, any, error) {
+		var s map[string]ContentTypeConfig
+		c := DefaultContentTypes
+		m, err := maps.ToStringMapE(v)
+		if err != nil {
+			return c, nil, err
+		}
+		if len(m) == 0 {
+			s = defaultContentTypesConfig
+		} else {
+			s = make(map[string]ContentTypeConfig)
+			m = maps.CleanConfigStringMap(m)
+			for k, v := range m {
+				var ctc ContentTypeConfig
+				if err := mapstructure.WeakDecode(v, &ctc); err != nil {
+					return c, nil, err
+				}
+				s[k] = ctc
+			}
+		}
+
+		for k := range s {
+			mediaType, found := types.GetByType(k)
+			if !found {
+				return c, nil, fmt.Errorf("unknown media type %q", k)
+			}
+			c.types = append(c.types, mediaType)
+		}
+
+		c.init(types)
+
+		return c, s, nil
+	}
+
+	ns, err := config.DecodeNamespace[map[string]ContentTypeConfig](in, buildConfig)
+	if err != nil {
+		return nil, fmt.Errorf("failed to decode media types: %w", err)
+	}
+	return ns, nil
+}
+
 // DecodeTypes decodes the given map of media types.
 func DecodeTypes(in map[string]any) (*config.ConfigNamespace[map[string]MediaTypeConfig, Types], error) {
 	buildConfig := func(v any) (Types, any, error) {
@@ -220,6 +273,6 @@ func DecodeTypes(in map[string]any) (*config.ConfigNamespace[map[string]MediaTyp
 // TODO(bep) get rid of this.
 var DefaultPathParser = &paths.PathParser{
 	IsContentExt: func(ext string) bool {
-		return DefaultContentTypes.IsContentSuffix(ext)
+		panic("not supported")
 	},
 }