diff --git a/CachingThumbnailer.go b/CachingThumbnailer.go
index 390408f..fa508b9 100644
--- a/CachingThumbnailer.go
+++ b/CachingThumbnailer.go
@@ -1,6 +1,9 @@
 package thumbnail
 
 import (
+	"mime"
+	"path/filepath"
+
 	lru "github.com/hashicorp/golang-lru"
 )
 
@@ -9,6 +12,8 @@ type CachingThumbnailer struct {
 	cache *lru.Cache // threadsafe, might be nil
 }
 
+var _ Thumbnailer = &CachingThumbnailer{} // interface assertion
+
 func NewCachingThumbnailer(cacheSize int, opts *Config) (Thumbnailer, error) {
 
 	upstream := NewThumbnailerEx(opts)
@@ -29,7 +34,11 @@ func NewCachingThumbnailer(cacheSize int, opts *Config) (Thumbnailer, error) {
 }
 
 func (this *CachingThumbnailer) RenderFile(absPath string) ([]byte, error) {
+	mimeType := mime.TypeByExtension(filepath.Ext(absPath))
+	return this.RenderFileAs(absPath, mimeType)
+}
 
+func (this *CachingThumbnailer) RenderFileAs(absPath, mimeType string) ([]byte, error) {
 	thumb, ok := this.cache.Get(absPath)
 	if ok {
 		return thumb.([]byte), nil
diff --git a/DirectThumbnailer.go b/DirectThumbnailer.go
index e78cd3a..864d840 100644
--- a/DirectThumbnailer.go
+++ b/DirectThumbnailer.go
@@ -11,6 +11,8 @@ type DirectThumbnailer struct {
 	cfg Config
 }
 
+var _ Thumbnailer = &DirectThumbnailer{} // interface assertion
+
 func NewThumbnailer(w, h int) Thumbnailer {
 	opts := DefaultConfig
 	opts.Width = w
@@ -29,6 +31,10 @@ func NewThumbnailerEx(opts *Config) Thumbnailer {
 
 func (this *DirectThumbnailer) RenderFile(absPath string) ([]byte, error) {
 	mimeType := mime.TypeByExtension(filepath.Ext(absPath))
+	return this.RenderFileAs(absPath, mimeType)
+}
+
+func (this *DirectThumbnailer) RenderFileAs(absPath, mimeType string) ([]byte, error) {
 
 	// Decode source
 
diff --git a/Thumbnailer.go b/Thumbnailer.go
index 0643023..c04bfc6 100644
--- a/Thumbnailer.go
+++ b/Thumbnailer.go
@@ -12,6 +12,7 @@ var (
 
 type Thumbnailer interface {
 	RenderFile(absPath string) ([]byte, error)
+	RenderFileAs(absPath, mimeType string) ([]byte, error)
 }
 
 func FiletypeSupported(ext string) bool {