diff --git a/src/beautifiers/index.coffee b/src/beautifiers/index.coffee
index 92acc76..e16d1a7 100644
--- a/src/beautifiers/index.coffee
+++ b/src/beautifiers/index.coffee
@@ -282,6 +282,7 @@ module.exports = class Beautifiers extends EventEmitter
         logger.verbose(allOptions)
 
         language ?= @getLanguage(grammar, filePath)
+        fileExtension = @getExtension(filePath)
 
         # Check if unsupported language
         if !language
@@ -343,6 +344,7 @@ module.exports = class Beautifiers extends EventEmitter
 
             context =
               filePath: filePath
+              fileExtension: fileExtension
 
             startTime = new Date()
             beautifier.loadExecutables()
@@ -405,7 +407,6 @@ module.exports = class Beautifiers extends EventEmitter
           if atom.config.get("atom-beautify.general.muteUnsupportedLanguageErrors")
             return resolve( null )
           else
-            fileExtension = @getExtension(filePath)
             repoBugsUrl = pkg.bugs.url
             title = "Atom Beautify could not find a supported beautifier for this file"
             detail = """
diff --git a/src/beautifiers/uncrustify/index.coffee b/src/beautifiers/uncrustify/index.coffee
index ffaa46a..e2c878c 100644
--- a/src/beautifiers/uncrustify/index.coffee
+++ b/src/beautifiers/uncrustify/index.coffee
@@ -46,7 +46,9 @@ module.exports = class Uncrustify extends Beautifier
     Arduino: true
   }
 
-  beautify: (text, language, options) ->
+  beautify: (text, language, options, context) ->
+    fileExtension = context.fileExtension
+
     uncrustify = @exe("uncrustify")
     # console.log('uncrustify.beautify', language, options)
     return new @Promise((resolve, reject) ->
@@ -99,9 +101,9 @@ module.exports = class Uncrustify extends Beautifier
         "-c"
         configPath
         "-f"
-        @tempFile("input", text)
+        @tempFile("input", text, fileExtension and ".#{fileExtension}")
         "-o"
-        outputFile = @tempFile("output", text)
+        outputFile = @tempFile("output", text, fileExtension and ".#{fileExtension}")
         "-l"
         lang
         ])