diff --git a/packer/cache.go b/packer/cache.go
index 6c2ed0207..9d69f0c56 100644
--- a/packer/cache.go
+++ b/packer/cache.go
@@ -4,6 +4,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"path/filepath"
+	"regexp"
 	"sync"
 )
 
@@ -14,6 +15,9 @@ type Cache interface {
 	// Packer guarantees that no other process will write to this file while
 	// the lock is held.
 	//
+	// If the key has an extension (e.g., file.ext), the resulting path
+	// will have that extension as well.
+	//
 	// The cache will block and wait for the lock.
 	Lock(string) string
 
@@ -68,7 +72,13 @@ func (f *FileCache) RUnlock(key string) {
 }
 
 func (f *FileCache) cachePath(key string, hashKey string) string {
-	return filepath.Join(f.CacheDir, hashKey)
+	var suffixPattern = regexp.MustCompile(`(\.\w+)$`)
+	matches := suffixPattern.FindStringSubmatch(key)
+	suffix := ""
+	if matches != nil {
+		suffix = matches[0]
+	}
+	return filepath.Join(f.CacheDir, hashKey+suffix)
 }
 
 func (f *FileCache) hashKey(key string) string {
diff --git a/packer/cache_test.go b/packer/cache_test.go
index 1be344ecc..5fe6f50d0 100644
--- a/packer/cache_test.go
+++ b/packer/cache_test.go
@@ -3,6 +3,7 @@ package packer
 import (
 	"io/ioutil"
 	"os"
+	"strings"
 	"testing"
 )
 
@@ -36,19 +37,23 @@ func TestFileCache(t *testing.T) {
 	defer os.RemoveAll(cacheDir)
 
 	cache := &FileCache{CacheDir: cacheDir}
-	path := cache.Lock("foo")
+	path := cache.Lock("foo.iso")
+
+	if !strings.HasSuffix(path, ".iso") {
+		t.Fatalf("path doesn't end with suffix '%s': '%s'", ".iso", path)
+	}
 	err = ioutil.WriteFile(path, []byte("data"), 0666)
 	if err != nil {
 		t.Fatalf("error writing: %s", err)
 	}
 
-	cache.Unlock("foo")
+	cache.Unlock("foo.iso")
 
-	path, ok := cache.RLock("foo")
+	path, ok := cache.RLock("foo.iso")
 	if !ok {
 		t.Fatal("cache says key doesn't exist")
 	}
-	defer cache.RUnlock("foo")
+	defer cache.RUnlock("foo.iso")
 
 	data, err := ioutil.ReadFile(path)
 	if err != nil {