diff --git a/src/calibre/srv/content.py b/src/calibre/srv/content.py
index 1432cc9611..bedcb129e6 100644
--- a/src/calibre/srv/content.py
+++ b/src/calibre/srv/content.py
@@ -159,7 +159,7 @@ def cover(ctx, rd, library_id, db, book_id, width=None, height=None):
 def book_filename(rd, book_id, mi, fmt):
     au = authors_to_string(mi.authors or [_('Unknown')])
     title = mi.title or _('Unknown')
-    ext = fmt
+    ext = fmt.lower()
     if ext == 'kepub' and 'Kobo Touch' in rd.inheaders.get('User-Agent', ''):
         ext = 'kepub.epub'
     fname = '%s - %s_%s.%s' % (title[:30], au[:30], book_id, ext)