diff --git a/lib/preprocessor.js b/lib/preprocessor.js
index b7b1a41b2..88f3c67cd 100644
--- a/lib/preprocessor.js
+++ b/lib/preprocessor.js
@@ -1,9 +1,12 @@
+var path = require('path');
 var fs = require('graceful-fs');
 var crypto = require('crypto');
 var mm = require('minimatch');
 
 var log = require('./logger').create('preprocess');
 
+// TODO(vojta): extract get/create temp dir somewhere else (use the same for launchers etc)
+var TMP = process.env.TMPDIR || process.env.TMP || process.env.TEMP || '/tmp';
 
 var sha1 = function(data) {
   var hash = crypto.createHash('sha1');
@@ -20,6 +23,7 @@ var createPreprocessor = function(config, basePath, injector) {
     var preprocessors = [];
     var nextPreprocessor = function(content) {
       if (!preprocessors.length) {
+        file.contentPath = path.normalize(TMP + '/'  + sha1(file.path) + path.extname(file.path));
         return fs.writeFile(file.contentPath, content, function() {
           done();
         });
@@ -56,11 +60,6 @@ var createPreprocessor = function(config, basePath, injector) {
 
     if (preprocessors.length) {
       return fs.readFile(file.originalPath, function(err, buffer) {
-        // TODO(vojta): extract get/create temp dir somewhere else (use the same for launchers etc)
-        var env = process.env;
-        var tmp = env.TMPDIR || env.TMP || env.TEMP || '/tmp';
-        file.contentPath = tmp + '/'  + sha1(file.originalPath) + '.js';
-
         nextPreprocessor(buffer.toString());
       });
     }
diff --git a/test/unit/preprocessor.spec.coffee b/test/unit/preprocessor.spec.coffee
index 936296587..3e0e12bbe 100644
--- a/test/unit/preprocessor.spec.coffee
+++ b/test/unit/preprocessor.spec.coffee
@@ -11,18 +11,25 @@ describe 'preprocessor', ->
     mockFs = mocks.fs.create
       some:
         'a.js': mocks.fs.file 0, 'content'
+        'style.less': mocks.fs.file 0, 'whatever'
+      temp: {} # so that we can write preprocessed content here
+
 
     mocks_ =
       'graceful-fs': mockFs
       minimatch: require 'minimatch'
 
-    m = mocks.loadFile __dirname + '/../../lib/preprocessor.js', mocks_
+    globals_ =
+      process:
+        env: TMPDIR: '/temp'
+        nextTick: process.nextTick
+
+    m = mocks.loadFile __dirname + '/../../lib/preprocessor.js', mocks_, globals_
 
 
   it 'should preprocess matching file', (done) ->
     fakePreprocessor = sinon.spy (content, file, done) ->
       file.path = file.path + '-preprocessed'
-      file.contentPath = '/some/new.js'
       done 'new-content'
 
     injector = new di.Injector [{'preprocessor:fake': ['factory', -> fakePreprocessor]}]
@@ -33,7 +40,7 @@ describe 'preprocessor', ->
     pp file, ->
       expect(fakePreprocessor).to.have.been.called
       expect(file.path).to.equal 'path-preprocessed'
-      expect(mockFs.readFileSync('/some/new.js').toString()).to.equal 'new-content'
+      expect(mockFs.readFileSync(file.contentPath).toString()).to.equal 'new-content'
       done()
 
 
@@ -53,7 +60,6 @@ describe 'preprocessor', ->
 
   it 'should apply all preprocessors', (done) ->
     fakePreprocessor1 = sinon.spy (content, file, done) ->
-      file.contentPath = '/some/new.js'
       file.path = file.path + '-p1'
       done content + '-c1'
 
@@ -74,5 +80,22 @@ describe 'preprocessor', ->
       expect(fakePreprocessor1).to.have.been.calledOnce
       expect(fakePreprocessor2).to.have.been.calledOnce
       expect(file.path).to.equal 'path-p1-p2'
-      expect(mockFs.readFileSync('/some/new.js').toString()).to.equal 'content-c1-c2'
+      expect(mockFs.readFileSync(file.contentPath).toString()).to.equal 'content-c1-c2'
+      done()
+
+
+  it 'should keep processed extension', (done) ->
+    fakePreprocessor = sinon.spy (content, file, done) ->
+      file.path = file.path.replace '.less', '.css'
+      done content
+
+    injector = new di.Injector [{
+      'preprocessor:less': ['factory', -> fakePreprocessor]
+    }]
+
+    pp = m.createPreprocessor {'**/*.less': ['less']}, null, injector
+    file = {originalPath: '/some/style.less', path: '/some/style.less'}
+
+    pp file, ->
+      expect(file.contentPath).to.match /\.css$/
       done()