diff --git a/interface.go b/extension.go
similarity index 59%
rename from interface.go
rename to extension.go
index bd8de15..91e184c 100644
--- a/interface.go
+++ b/extension.go
@@ -1,10 +1,5 @@
 package goldsmith
 
-import (
-	"io"
-	"time"
-)
-
 // Plugin contains the minimum set of methods required on plugins. Plugins can
 // also optionally implement Initializer, Processor, and Finalizer interfaces.
 type Plugin interface {
@@ -33,33 +28,3 @@ type Filter interface {
 	Name() string
 	Accept(file File) bool
 }
-
-type (
-	FileProp  any
-	FileProps map[string]FileProp
-)
-
-// File represents in-memory or on-disk files in a chain.
-type File interface {
-	Path() string
-	Dir() string
-	Name() string
-	Ext() string
-	Rename(path string) error
-
-	Size() int64
-	ModTime() time.Time
-
-	Read(data []byte) (int, error)
-	WriteTo(writer io.Writer) (int64, error)
-	Seek(offset int64, whence int) (int64, error)
-
-	SetProp(name string, value FileProp)
-	Prop(name string) (FileProp, bool)
-	PropOrDef(name string, valueDef FileProp) FileProp
-	Props() FileProps
-	CopyProps(file File)
-	RemoveProp(name string)
-
-	GoString() string
-}
diff --git a/file.go b/file.go
new file mode 100644
index 0000000..badb6aa
--- /dev/null
+++ b/file.go
@@ -0,0 +1,36 @@
+package goldsmith
+
+import (
+	"io"
+	"time"
+)
+
+type (
+	FileProp  any
+	FileProps map[string]FileProp
+)
+
+// File represents in-memory or on-disk files in a chain.
+type File interface {
+	Path() string
+	Dir() string
+	Name() string
+	Ext() string
+	Rename(path string) error
+
+	Size() int64
+	ModTime() time.Time
+
+	Read(data []byte) (int, error)
+	WriteTo(writer io.Writer) (int64, error)
+	Seek(offset int64, whence int) (int64, error)
+
+	SetProp(name string, value FileProp)
+	Prop(name string) (FileProp, bool)
+	PropOrDef(name string, valueDef FileProp) FileProp
+	Props() FileProps
+	CopyProps(file File)
+	RemoveProp(name string)
+
+	GoString() string
+}