diff --git a/odoo/addons/base/ir/ir_http.py b/odoo/addons/base/ir/ir_http.py
index c01e6d4a4ea..179f0b4e935 100644
--- a/odoo/addons/base/ir/ir_http.py
+++ b/odoo/addons/base/ir/ir_http.py
@@ -321,9 +321,11 @@ class IrHttp(models.AbstractModel):
                 mimetype = guess_mimetype(base64.b64decode(content), default=default_mimetype)
 
         # extension
-        extension = mimetypes.guess_extension(mimetype)
-        if extension and not filename.endswith(extension):
-            filename = "%s%s" % (filename, extension)
+        _, existing_extension = os.path.splitext(filename)
+        if not existing_extension:
+            extension = mimetypes.guess_extension(mimetype)
+            if extension:
+                filename = "%s%s" % (filename, extension)
 
         headers += [('Content-Type', mimetype), ('X-Content-Type-Options', 'nosniff')]