diff --git a/CHANGELOG.md b/CHANGELOG.md
index a1b8d6dc..444bcba4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 # Changelog
 
+## Unreleased
+
 ## 1.13.1 - 2020-03-01
 ### Additions
 - [hentaihand] add extractors ([#605](https://github.com/mikf/gallery-dl/issues/605))
diff --git a/gallery_dl/downloader/http.py b/gallery_dl/downloader/http.py
index 7d4d7d59..844e422c 100644
--- a/gallery_dl/downloader/http.py
+++ b/gallery_dl/downloader/http.py
@@ -193,6 +193,9 @@ class HttpDownloader(DownloaderBase):
         mtype = response.headers.get("Content-Type", "image/jpeg")
         mtype = mtype.partition(";")[0]
 
+        if "/" not in mtype:
+            mtype = "image/" + mtype
+
         if mtype in MIMETYPE_MAP:
             return MIMETYPE_MAP[mtype]
 
diff --git a/gallery_dl/version.py b/gallery_dl/version.py
index 719595e3..6f3807b2 100644
--- a/gallery_dl/version.py
+++ b/gallery_dl/version.py
@@ -6,4 +6,4 @@
 # it under the terms of the GNU General Public License version 2 as
 # published by the Free Software Foundation.
 
-__version__ = "1.13.1"
+__version__ = "1.13.2-dev"