Unknown file error

[Stephensonel]

I have upgraded iOS on my iPhone6. But after upgrading i cannot open one very important .OST extension file which was emailed to me a month ago.
Getting the message: Unknown error
Can anyone explain whats happening and provide a solution? I really need to open this file!

 

[Tartarus]

Did you try opening that email on an other device? It via the web interface of the email client?