diff --git a/kittengroomer/helpers.py b/kittengroomer/helpers.py
index 9604c49..10dd6a9 100644
--- a/kittengroomer/helpers.py
+++ b/kittengroomer/helpers.py
@@ -45,9 +45,13 @@ class FileBase(object):
         self.dst_path = dst_path
         self.log_details = {'filepath': self.src_path}
         self.log_string = ''
-        _, self.extension = os.path.splitext(self.src_path)
+        self._determine_extension()
         self._determine_mimetype()
 
+    def _determine_extension(self):
+        _, ext = os.path.splitext(self.src_path)
+        self.extension = ext.lower()
+
     def _determine_mimetype(self):
         if os.path.islink(self.src_path):
             # magic will throw an IOError on a broken symlink
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index f59c862..0d0c338 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -113,6 +113,13 @@ class TestFileBase:
         # assert file.log_details == copied_log     # this fails for now, we need to make log_details undeletable
         # we should probably check for more extensions here
 
+    def test_extension_uppercase(self, tmpdir):
+        file_path = tmpdir.join('TEST.TXT')
+        file_path.write('testing')
+        file_path = file_path.strpath
+        file = FileBase(file_path, file_path)
+        assert file.extension == '.txt'
+
     def test_mimetypes(self, generic_conf_file):
         assert generic_conf_file.has_mimetype()
         assert generic_conf_file.mimetype == 'text/plain'