diff --git a/src/Handler/Utils/Form.hs b/src/Handler/Utils/Form.hs
index fcd084a43..937d3eec3 100644
--- a/src/Handler/Utils/Form.hs
+++ b/src/Handler/Utils/Form.hs
@@ -676,7 +676,7 @@ zipFileField doUnpack permittedExtensions = Field{..}
     fieldEnctype = Multipart
     fieldParse _ files
       | [f@FileInfo{..}] <- files
-      , maybe True (anyOf (re _nullable . folded . unpacked) (`isExtensionOf` unpack fileName)) permittedExtensions || doUnpack
+      , maybe True (anyOf (re _nullable . folded . unpacked) ((flip isExtensionOf `on` CI.foldCase) $ unpack fileName)) permittedExtensions || doUnpack
       = return . Right . Just $ bool (yieldM . acceptFile) sourceFiles doUnpack f
       | null files   = return $ Right Nothing
       | otherwise    = return . Left $ SomeMessage MsgOnlyUploadOneFile