Unable to download Gmail attachments from Firefox browser

I am unable to download Gmail attachments, they are getting downloaded without extension and this is happening since last few days.

This didn't use to happen before, tried and tested with multiple emails and multiple attachments, if I refresh the browser and then try immediately it works correctly, but if I leave the browser idle or navitgate through multiple emails then it gets downloaded without extenstion. Tried clearing cache for all websites, still it persists.