[simple-scan] Fix documents always being saved as JPEG.

[Robert Ancell <rancell src gnome org>]

commit 62bd71f794cf4aeb54d71cf7065ad2b69e43bd23
Author: Robert Ancell <robert ancell canonical com>
Date:   Mon Apr 6 10:46:33 2020 +1200

    Fix documents always being saved as JPEG.
    
    Was an off-by-one error in ca296b2d
    
    Fixes #162

 src/app-window.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/app-window.vala b/src/app-window.vala
index 13598ad..3ac66ce 100644
--- a/src/app-window.vala
+++ b/src/app-window.vala
@@ -683,7 +683,7 @@ public class AppWindow : Gtk.ApplicationWindow
         var extension_index = uri.last_index_of_char ('.');
         if (extension_index < 0)
             return "image/jpeg";
-        var extension = uri.substring (extension_index);
+        var extension = uri.substring (extension_index + 1);
 
         var mime_type = extension_to_mime_type (extension);
         if (mime_type == null)