adding extesion fallback flag in jam pack

pack.go

@@ -69,6 +69,7 @@ type PackBuild struct {
 	noColor bool
 
 	buildpacks    []string
+	extensions    []string
 	network       string
 	builder       string
 	clearCache    bool
@@ -88,6 +89,11 @@ func (pb PackBuild) WithBuildpacks(buildpacks ...string) PackBuild {
 	return pb
 }
 
+func (pb PackBuild) WithExtensions(extensions ...string) PackBuild {
+	pb.extensions = append(pb.extensions, extensions...)
+	return pb
+}
+
 func (pb PackBuild) WithNetwork(name string) PackBuild {
 	pb.network = name
 	return pb
@@ -156,6 +162,10 @@ func (pb PackBuild) Execute(name, path string) (Image, fmt.Stringer, error) {
 		args = append(args, "--buildpack", buildpack)
 	}
 
+	for _, extension := range pb.extensions {
+		args = append(args, "--extension", extension)
+	}
+
 	if pb.network != "" {
 		args = append(args, "--network", pb.network)
 	}

packagers/jam.go

@@ -1,6 +1,7 @@
 package packagers
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 
@@ -32,9 +33,21 @@ func (j Jam) WithExecutable(executable Executable) Jam {
 }
 
 func (j Jam) Execute(buildpackDir, output, version string, offline bool) error {
+
+	extensionTomlPath := filepath.Join(buildpackDir, "extension.toml")
+
+	buildpackOrExtensionToml := "buildpack.toml"
+	command := "--buildpack"
+	if _, err := os.Stat(extensionTomlPath); err == nil {
+		buildpackOrExtensionToml = "extension.toml"
+		command = "--extension"
+	}
+
+	fmt.Println("jam", command, filepath.Join(buildpackDir, buildpackOrExtensionToml), "--output", output, "--version", version, "--offline", offline)
+
 	args := []string{
 		"pack",
-		"--buildpack", filepath.Join(buildpackDir, "buildpack.toml"),
+		command, filepath.Join(buildpackDir, buildpackOrExtensionToml),
 		"--output", output,
 		"--version", version,
 	}