diff --git a/builder/virtualbox/iso/builder.go b/builder/virtualbox/iso/builder.go
index d095eae2d..b6ca982fd 100644
--- a/builder/virtualbox/iso/builder.go
+++ b/builder/virtualbox/iso/builder.go
@@ -230,6 +230,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 			Description:  "ISO",
 			ResultKey:    "iso_path",
 			Url:          b.config.ISOUrls,
+			Extension:    "iso",
 		},
 		&vboxcommon.StepOutputDir{
 			Force: b.config.PackerForce,