Trouble opening file on kindle

I was just wondering if anyone could help me with this problem.

I've formatted my file in calibre and saved it as a MOBI file. When I look at it in the kindle previewer it works fine, but when I try to open it it doesn't work.
I can click on the file and download it. And it says the download was successful but then I can't find it on my kindle.

Does anyone know what I'm doing wrong?