Add mime.type for yaml

According to
https://www.ietf.org/archive/id/draft-ietf-httpapi-yaml-mediatypes-00.html,
we should be using `application/yaml`, but since GCS uses
application/x-yaml, and we emulate GCS, we stick to what GCS' behavior.

Closes #990.

main.go

@@ -33,6 +33,7 @@ func main() {
 	var emptyBuckets []string
 	opts := cfg.ToFakeGcsOptions()
 	if cfg.Seed != "" {
+		addMimeTypes()
 		opts.InitialObjects, emptyBuckets = generateObjectsFromFiles(logger, cfg.Seed)
 	}
 
@@ -50,6 +51,16 @@ func main() {
 	<-ch
 }
 
+func addMimeTypes() {
+	mapping := map[string]string{
+		".yaml": "application/x-yaml",
+		".yml":  "application/x-yaml",
+	}
+	for ext, typ := range mapping {
+		mime.AddExtensionType(ext, typ)
+	}
+}
+
 func generateObjectsFromFiles(logger *logrus.Logger, folder string) ([]fakestorage.Object, []string) {
 	var objects []fakestorage.Object
 	var emptyBuckets []string