diff --git a/images/custom-error-pages/main.go b/images/custom-error-pages/main.go
index 8f4e6b267..7d3d73029 100644
--- a/images/custom-error-pages/main.go
+++ b/images/custom-error-pages/main.go
@@ -62,6 +62,12 @@ const (
 	// ErrFilesPathVar is the name of the environment variable indicating
 	// the location on disk of files served by the handler.
 	ErrFilesPathVar = "ERROR_FILES_PATH"
+
+	// DefaultFormatVar is the name of the environment variable indicating
+	// the default error MIME type that should be returned if either the
+	// client does not specify an Accept header, or the Accept header provided
+	// cannot be mapped to a file extension.
+	DefaultFormatVar = "DEFAULT_RESPONSE_FORMAT"
 )
 
 func init() {
@@ -75,7 +81,12 @@ func main() {
 		errFilesPath = os.Getenv(ErrFilesPathVar)
 	}
 
-	http.HandleFunc("/", errorHandler(errFilesPath))
+	defaultFormat := "text/html"
+	if os.Getenv(DefaultFormatVar) != "" {
+		defaultFormat = os.Getenv(DefaultFormatVar)
+	}
+
+	http.HandleFunc("/", errorHandler(errFilesPath, defaultFormat))
 
 	http.Handle("/metrics", promhttp.Handler())
 
@@ -86,10 +97,16 @@ func main() {
 	http.ListenAndServe(fmt.Sprintf(":8080"), nil)
 }
 
-func errorHandler(path string) func(http.ResponseWriter, *http.Request) {
+func errorHandler(path, defaultFormat string) func(http.ResponseWriter, *http.Request) {
+	defaultExts, err := mime.ExtensionsByType(defaultFormat)
+	if err != nil || len(defaultExts) == 0 {
+		panic("couldn't get file extension for default format")
+	}
+	defaultExt := defaultExts[0]
+
 	return func(w http.ResponseWriter, r *http.Request) {
 		start := time.Now()
-		ext := "html"
+		ext := defaultExt
 
 		if os.Getenv("DEBUG") != "" {
 			w.Header().Set(FormatHeader, r.Header.Get(FormatHeader))
@@ -105,14 +122,14 @@ func errorHandler(path string) func(http.ResponseWriter, *http.Request) {
 
 		format := r.Header.Get(FormatHeader)
 		if format == "" {
-			format = "text/html"
+			format = defaultFormat
 			log.Printf("format not specified. Using %v", format)
 		}
 
 		cext, err := mime.ExtensionsByType(format)
 		if err != nil {
 			log.Printf("unexpected error reading media type extension: %v. Using %v", err, ext)
-			format = "text/html"
+			format = defaultFormat
 		} else if len(cext) == 0 {
 			log.Printf("couldn't get media type extension. Using %v", ext)
 		} else {
@@ -131,6 +148,10 @@ func errorHandler(path string) func(http.ResponseWriter, *http.Request) {
 		if !strings.HasPrefix(ext, ".") {
 			ext = "." + ext
 		}
+		// special case for compatibility
+		if ext == ".htm" {
+			ext = ".html"
+		}
 		file := fmt.Sprintf("%v/%v%v", path, code, ext)
 		f, err := os.Open(file)
 		if err != nil {