diff --git a/app/models/optimized_image.rb b/app/models/optimized_image.rb
index 3c944959a6..6a7c388f38 100644
--- a/app/models/optimized_image.rb
+++ b/app/models/optimized_image.rb
@@ -123,7 +123,7 @@ class OptimizedImage < ActiveRecord::Base
   def self.prepend_decoder!(path)
     extension = File.extname(path)[1..-1]
     raise Discourse::InvalidAccess unless extension[IM_DECODERS]
-    path = "#{extension}:#{path}"
+    "#{extension}:#{path}"
   end
 
   def self.thumbnail_or_resize
@@ -133,8 +133,8 @@ class OptimizedImage < ActiveRecord::Base
   def self.resize_instructions(from, to, dimensions, opts = {})
     ensure_safe_paths!(from, to)
 
-    prepend_decoder!(from)
-    prepend_decoder!(to)
+    from = prepend_decoder!(from)
+    to = prepend_decoder!(to)
 
     # NOTE: ORDER is important!
     %W{
@@ -170,8 +170,8 @@ class OptimizedImage < ActiveRecord::Base
   def self.crop_instructions(from, to, dimensions, opts = {})
     ensure_safe_paths!(from, to)
 
-    prepend_decoder!(from)
-    prepend_decoder!(to)
+    from = prepend_decoder!(from)
+    to = prepend_decoder!(to)
 
     %W{
       convert
@@ -205,8 +205,8 @@ class OptimizedImage < ActiveRecord::Base
   def self.downsize_instructions(from, to, dimensions, opts = {})
     ensure_safe_paths!(from, to)
 
-    prepend_decoder!(from)
-    prepend_decoder!(to)
+    from = prepend_decoder!(from)
+    to = prepend_decoder!(to)
 
     %W{
       convert
diff --git a/lib/upload_creator.rb b/lib/upload_creator.rb
index 5f00da8e7f..c48ae053d4 100644
--- a/lib/upload_creator.rb
+++ b/lib/upload_creator.rb
@@ -140,8 +140,8 @@ class UploadCreator
 
     OptimizedImage.ensure_safe_paths!(from, to)
 
-    OptimizedImage.prepend_decoder!(from)
-    OptimizedImage.prepend_decoder!(to)
+    from = OptimizedImage.prepend_decoder!(from)
+    to = OptimizedImage.prepend_decoder!(to)
 
     begin
       execute_convert(from, to)
@@ -220,7 +220,7 @@ class UploadCreator
     path = @file.path
 
     OptimizedImage.ensure_safe_paths!(path)
-    OptimizedImage.prepend_decoder!(path)
+    path = OptimizedImage.prepend_decoder!(path)
 
     Discourse::Utils.execute_command('convert', path, '-auto-orient', path)