Add extension to outname if required

slider/report.py

@@ -38,6 +38,9 @@ def report_reg(dirs, out, embed=False):
         images=images,
     )
 
+    if not out.endswith('.html'):
+        out = out + '.html'
+
     with open(out, "w") as outfile:
         outfile.write(html)