HD Kindle Fire HD 8th gen unable to open mobi or AZW3

[amanda138a]

I had to buy a new kindle fire, it is a Kindle Fire 8 HD 8th generation (6.3.0.1)

Sometimes when I move books across either by cable (via send to device) or the calibre companion, if the book is not on showing in the library then I could access via Documents, (or if I was to download a file direct to the kindle fire)

However now, regardless whether the file is mobi or AZW3 I get a message

"Sorry, no application available"

This is ok if I used the calibre companion I can access through it (though I have to remember to bookmark it otherwise i lose my place every time!!! But if I have sent it via USB cable or downloaded it I am stuck!

Anyone have this same problem or know how I can get around it?