--- a/src/pyams_media/media.py	Mon Sep 11 13:43:22 2017 +0200
+++ b/src/pyams_media/media.py	Mon Sep 11 13:44:23 2017 +0200
@@ -84,10 +84,15 @@
         alsoProvides(target, IMediaConversion)
         if extension is None:
             extension = guess_extension(format)
-        target_name = '{name}{width}.{extension}'.format(name=target.content_type.decode().split('/', 1)[0]
-                                                             if target.content_type else 'media',
+        content_type = target.content_type
+        if isinstance(content_type, bytes):
+            content_type = content_type.decode()
+        target_name = '{name}{width}.{extension}'.format(name=content_type.split('/', 1)[0]
+                                                             if content_type else 'media',
                                                          width='-{0}'.format(width) if width else '',
                                                          extension=extension)
+        if target_name in self:
+            del self[target_name]
         target.filename = target_name
         self[target_name] = target
         return target