Return md5sum and filesize in the list of images

Ref https://github.com/GNS3/gns3-gui/issues/1590

gns3server/utils/images.py

@@ -92,7 +92,9 @@ def md5sum(path):
 
     try:
         with open(path + '.md5sum') as f:
-            return f.read()
+            md5 = f.read()
+            if len(md5) == 32:
+                return md5
     # Unicode error is when user rename an image to .md5sum ....
     except (OSError, UnicodeDecodeError):
         pass