Register all media types when in server mode

Fixes #3274

hugolib/site.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"html/template"
 	"io"
+	"mime"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -610,6 +611,15 @@ type whatChanged struct {
 	other  bool
 }
 
+// RegisterMediaTypes will register the Site's media types in the mime
+// package, so it will behave correctly with Hugo's built-in server.
+func (s *Site) RegisterMediaTypes() {
+	for _, mt := range s.mediaTypesConfig {
+		// The last one will win if there are any duplicates.
+		mime.AddExtensionType("."+mt.Suffix, mt.Type()+"; charset=utf-8")
+	}
+}
+
 // reBuild partially rebuilds a site given the filesystem events.
 // It returns whetever the content source was changed.
 func (s *Site) reProcess(events []fsnotify.Event) (whatChanged, error) {