I was getting this a lot when I used chrome browser.
WiFi or mobile data.
I found that by clearing the cache and cookies it would be om for a few days to weeks, then slowly start again.
hope this helps.
I can't upload anymore. I could off this phone. Don't know how long it has been an issue (got a note 7 and stopped using this for a while). In the app, you press the image link and it comes up saying to select email client.