It sounds like there might either be an issue with the particular file or there's no app available on the phone that can handle the file format. A few things I'd suggest checking on as a start:
1) Do you see an error in-app when you click on the attachment or with the next step when you click 'View'?
2) Can you view the same attachment when you access the Trello card from a computer via web browser at https://trello.com/ or from a different phone?
3) What specific format is the file saved in?
Feel free to send in additional details to https://trello.com/contact so we can take a closer look if needed. Thanks!
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