Can't preview file upload in form entry

Hi, I apologise if I’m not posting this in the correct place but I’m a bit desperate for help! I have a gravity form here: http://www.newthorn.co.uk/register-online/ – there are a couple of fields where somebody can upload a file. It’s been working well for the last 2 or 3 years but now, although the file uploads are still being uploaded to the server perfectly well, when I try to view them in the WordPress Admin/Gravity Forms Form Entries, instead of previewing the file when the filename upload is clicked on, it opens a Save As dialog box. If you then save the file to a computer and try to open it it won’t open. There’s no issue with the host/server – I’ve checked with them and they suggest that the fault is with the plugin. Gravity Forms say it could be a problem with the theme or plugins but none of these have changed before or since the problems started. Any help or advice would be extremely, greatly appreciated. Many thanks. Kathryn

https://wordpress.org/plugins/gravity-forms-addons/