tpl/transform: Add transform.Unmarshal func

Fixes #5428

parser/metadecoders/format.go

@@ -17,6 +17,8 @@ import (
 	"path/filepath"
 	"strings"
 
+	"github.com/gohugoio/hugo/media"
+
 	"github.com/gohugoio/hugo/parser/pageparser"
 )
 
@@ -55,6 +57,18 @@ func FormatFromString(formatStr string) Format {
 
 }
 
+// FormatFromMediaType gets the Format given a MIME type, empty string
+// if unknown.
+func FormatFromMediaType(m media.Type) Format {
+	for _, suffix := range m.Suffixes {
+		if f := FormatFromString(suffix); f != "" {
+			return f
+		}
+	}
+
+	return ""
+}
+
 // FormatFromFrontMatterType will return empty if not supported.
 func FormatFromFrontMatterType(typ pageparser.ItemType) Format {
 	switch typ {
@@ -70,3 +84,39 @@ func FormatFromFrontMatterType(typ pageparser.ItemType) Format {
 		return ""
 	}
 }
+
+// FormatFromContentString tries to detect the format (JSON, YAML or TOML)
+// in the given string.
+// It return an empty string if no format could be detected.
+func FormatFromContentString(data string) Format {
+	jsonIdx := strings.Index(data, "{")
+	yamlIdx := strings.Index(data, ":")
+	tomlIdx := strings.Index(data, "=")
+
+	if isLowerIndexThan(jsonIdx, yamlIdx, tomlIdx) {
+		return JSON
+	}
+
+	if isLowerIndexThan(yamlIdx, tomlIdx) {
+		return YAML
+	}
+
+	if tomlIdx != -1 {
+		return TOML
+	}
+
+	return ""
+}
+
+func isLowerIndexThan(first int, others ...int) bool {
+	if first == -1 {
+		return false
+	}
+	for _, other := range others {
+		if other != -1 && other < first {
+			return false
+		}
+	}
+
+	return true
+}