diff --git a/lib/trantor.go b/lib/trantor.go
index 24b34e5..0bd0320 100644
--- a/lib/trantor.go
+++ b/lib/trantor.go
@@ -113,6 +113,7 @@ func downloadHandler(h handler) {
 	defer f.Close()
 
 	addCacheControlHeader(h.w, false)
+	h.w.Header().Set("Content-Type", "application/epub+zip")
 	http.ServeContent(h.w, h.r, bookFileName(book), book.UploadDate, f)
 }