media, hugolib: Support extension-less media types

This change is motivated by Netlify's `_redirects` files, which is currently not possible to generate with Hugo.

This commit adds a `Delimiter` field to media type, which defaults to ".", but can be blanked out.

Fixes #3614

output/layout.go

@@ -181,17 +181,37 @@ func resolveListTemplate(d LayoutDescriptor, f Format,
 	case "taxonomyTerm":
 		layouts = resolveTemplate(taxonomyTermLayouts, d, f)
 	}
-
 	return layouts
 }
 
 func resolveTemplate(templ string, d LayoutDescriptor, f Format) []string {
+	delim := "."
+	if f.MediaType.Delimiter == "" {
+		delim = ""
+	}
 	layouts := strings.Fields(replaceKeyValues(templ,
-		"SUFFIX", f.MediaType.Suffix,
+		".SUFFIX", delim+f.MediaType.Suffix,
 		"NAME", strings.ToLower(f.Name),
 		"SECTION", d.Section))
 
-	return layouts
+	return filterDotLess(layouts)
+}
+
+func filterDotLess(layouts []string) []string {
+	var filteredLayouts []string
+
+	for _, l := range layouts {
+		// This may be constructed, but media types can be suffix-less, but can contain
+		// a delimiter.
+		l = strings.TrimSuffix(l, ".")
+		// If media type has no suffix, we have "index" type of layouts in this list, which
+		// doesn't make much sense.
+		if strings.Contains(l, ".") {
+			filteredLayouts = append(filteredLayouts, l)
+		}
+	}
+
+	return filteredLayouts
 }
 
 func prependTextPrefixIfNeeded(f Format, layouts ...string) []string {
@@ -220,7 +240,12 @@ func regularPageLayouts(types string, layout string, f Format) []string {
 		layout = "single"
 	}
 
-	suffix := f.MediaType.Suffix
+	delimiter := "."
+	if f.MediaType.Delimiter == "" {
+		delimiter = ""
+	}
+
+	suffix := delimiter + f.MediaType.Suffix
 	name := strings.ToLower(f.Name)
 
 	if types != "" {
@@ -229,15 +254,15 @@ func regularPageLayouts(types string, layout string, f Format) []string {
 		// Add type/layout.html
 		for i := range t {
 			search := t[:len(t)-i]
-			layouts = append(layouts, fmt.Sprintf("%s/%s.%s.%s", strings.ToLower(path.Join(search...)), layout, name, suffix))
-			layouts = append(layouts, fmt.Sprintf("%s/%s.%s", strings.ToLower(path.Join(search...)), layout, suffix))
+			layouts = append(layouts, fmt.Sprintf("%s/%s.%s%s", strings.ToLower(path.Join(search...)), layout, name, suffix))
+			layouts = append(layouts, fmt.Sprintf("%s/%s%s", strings.ToLower(path.Join(search...)), layout, suffix))
 
 		}
 	}
 
 	// Add _default/layout.html
-	layouts = append(layouts, fmt.Sprintf("_default/%s.%s.%s", layout, name, suffix))
-	layouts = append(layouts, fmt.Sprintf("_default/%s.%s", layout, suffix))
+	layouts = append(layouts, fmt.Sprintf("_default/%s.%s%s", layout, name, suffix))
+	layouts = append(layouts, fmt.Sprintf("_default/%s%s", layout, suffix))
 
-	return layouts
+	return filterDotLess(layouts)
 }

output/layout_test.go

@@ -21,14 +21,34 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-var ampType = Format{
-	Name:      "AMP",
-	MediaType: media.HTMLType,
-	BaseName:  "index",
-}
-
 func TestLayout(t *testing.T) {
 
+	noExtNoDelimMediaType := media.TextType
+	noExtNoDelimMediaType.Suffix = ""
+	noExtNoDelimMediaType.Delimiter = ""
+
+	noExtMediaType := media.TextType
+	noExtMediaType.Suffix = ""
+
+	var (
+		ampType = Format{
+			Name:      "AMP",
+			MediaType: media.HTMLType,
+			BaseName:  "index",
+		}
+
+		noExtDelimFormat = Format{
+			Name:      "NEM",
+			MediaType: noExtNoDelimMediaType,
+			BaseName:  "_redirects",
+		}
+		noExt = Format{
+			Name:      "NEX",
+			MediaType: noExtMediaType,
+			BaseName:  "next",
+		}
+	)
+
 	for _, this := range []struct {
 		name           string
 		d              LayoutDescriptor
@@ -39,6 +59,12 @@ func TestLayout(t *testing.T) {
 	}{
 		{"Home", LayoutDescriptor{Kind: "home"}, true, "", ampType,
 			[]string{"index.amp.html", "index.html", "_default/list.amp.html", "_default/list.html", "theme/index.amp.html", "theme/index.html"}},
+		{"Home, no ext or delim", LayoutDescriptor{Kind: "home"}, true, "", noExtDelimFormat,
+			[]string{"index.nem", "_default/list.nem"}},
+		{"Home, no ext", LayoutDescriptor{Kind: "home"}, true, "", noExt,
+			[]string{"index.nex", "_default/list.nex"}},
+		{"Page, no ext or delim", LayoutDescriptor{Kind: "page"}, true, "", noExtDelimFormat,
+			[]string{"_default/single.nem", "theme/_default/single.nem"}},
 		{"Section", LayoutDescriptor{Kind: "section", Section: "sect1"}, false, "", ampType,
 			[]string{"section/sect1.amp.html", "section/sect1.html"}},
 		{"Taxonomy", LayoutDescriptor{Kind: "taxonomy", Section: "tag"}, false, "", ampType,