diff --git a/byg/src/DependencyGenerator.hs b/byg/src/DependencyGenerator.hs
index be1c392..bfb2704 100644
--- a/byg/src/DependencyGenerator.hs
+++ b/byg/src/DependencyGenerator.hs
@@ -15,7 +15,7 @@ module DependencyGenerator
 
   , joinPaths
   , fileComponents
-  , isImageFilename
+  , isImageExtension
   , convertedImageFilename
   , applyTemplate
   , listDirectory
@@ -97,10 +97,9 @@ mapDepGenM_ f input = do
   _ <- mapDepGenM (\x -> f x >> pure NoToken) input
   pure ()
 
-filterDepGenM :: (Token a -> DepGenM' Bool) -> Token [a] -> DepGenM' [a]
-filterDepGenM f input = do
-  conds <- mapDepGenM f input
-  genDependency (makeDependency (TupleToken (input, conds)) FilterComp)
+filterDepGenM :: Token [Bool] -> Token [a] -> DepGenM' [a]
+filterDepGenM mask input = do
+  genDependency (makeDependency (TupleToken (input, mask)) FilterComp)
 
 unzipFstDepGenM :: TokenableTo [(a, b)] t => t -> DepGenM (Token [a])
 unzipFstDepGenM t = do
@@ -141,8 +140,8 @@ joinPaths (a, b) = do
 fileComponents :: TokenableTo FilePath a => a -> DepGenM' (String, String)
 fileComponents a = runFunction FileComponents =<< toToken a
 
-isImageFilename :: TokenableTo FilePath a => a -> DepGenM' Bool
-isImageFilename a = runFunction IsImageFilename =<< toToken a
+isImageExtension :: TokenableTo FilePath a => a -> DepGenM' Bool
+isImageExtension a = runFunction IsImageExtension =<< toToken a
 
 convertedImageFilename :: TokenableTo FilePath a => a -> DepGenM' FilePath
 convertedImageFilename a = runFunction ConvertedImageFilename =<< toToken a
diff --git a/byg/src/Evaluation/Function.hs b/byg/src/Evaluation/Function.hs
index efe27d4..81fbeff 100644
--- a/byg/src/Evaluation/Function.hs
+++ b/byg/src/Evaluation/Function.hs
@@ -9,7 +9,7 @@ evalFunction f x = case (f, x) of
   (JoinPaths, Tuple (String _, String _)) ->
     String undefined
 
-  (IsImageFilename, String _) ->
+  (IsImageExtension, String _) ->
     Bool undefined
 
   (ConvertedImageFilename, String _) ->
diff --git a/byg/src/SiteGenerator.hs b/byg/src/SiteGenerator.hs
index 23394de..833b13b 100644
--- a/byg/src/SiteGenerator.hs
+++ b/byg/src/SiteGenerator.hs
@@ -7,7 +7,8 @@ handleRecipeDir :: Token FilePath -> Token Template -> Token FilePath -> DepGenM
 handleRecipeDir outputDir template dir = do
   dirContents <- listDirectory dir
   (bases, exts) <- unzipDepGenM (mapDepGenM fileComponents dirContents)
-  imageFilenames <- filterDepGenM isImageFilename dirContents
+  areImageFilenames <- mapDepGenM isImageExtension exts
+  imageFilenames <- filterDepGenM areImageFilenames dirContents
   convertedImageFilenames <- mapDepGenM convertedImageFilename imageFilenames
   mapDepGenM_
     (\files -> convertImage (files, inject $ ResizeToWidth 800))
diff --git a/byg/src/Types/Function.hs b/byg/src/Types/Function.hs
index f7ff953..8921677 100644
--- a/byg/src/Types/Function.hs
+++ b/byg/src/Types/Function.hs
@@ -6,7 +6,7 @@ import Language.Haskell.TH.Syntax (Lift)
 
 data Function = JoinPaths
               | FileComponents
-              | IsImageFilename
+              | IsImageExtension
               | ConvertedImageFilename
               | ApplyTemplate
   deriving (Show, Lift)