diff --git a/flask_s3.py b/flask_s3.py
index 573cc9d..a4e80b9 100644
--- a/flask_s3.py
+++ b/flask_s3.py
@@ -4,7 +4,10 @@ import logging
 import os
 import re
 import gzip
-import cStringIO
+try:
+    from cStringIO import StringIO
+except ImportError:
+    from io import StringIO
 import mimetypes
 from collections import defaultdict
 
@@ -216,12 +219,16 @@ def _write_files(s3, app, static_url_loc, static_folder, files, bucket,
                     # When we use GZIP we have to explicitly set the content type
                     (mimetype, encoding) = mimetypes.guess_type(file_path,
                         False)
-                    h["content-type"] = mimetype
+                    if mimetype:
+                        h["content-type"] = mimetype
+                    else:
+                        logger.warn("Unable to detect mimetype for %s" %
+                            file_path)
 
             with open(file_path) as fp:
                 metadata, params = split_metadata_params(merge_two_dicts(app.config['S3_HEADERS'], h))
                 if should_gzip:
-                    compressed = cStringIO.StringIO()
+                    compressed = StringIO()
                     z = gzip.GzipFile(os.path.basename(file_path), 'wb', 9,
                         compressed)
                     z.write(fp.read())