There could be a few reasons why you can't upload attachments and you're seeing the exclamation point.
Unsupported file format. Double check you didn't use a file format Trello doesn't support.
You exceeded the file size. Trello has file limits for free accounts (10MB), and for paid accounts, the limit is 250 MB.
Trello may have gone down temporarily. If any incidents were reported with viewing and loading attachments, you can view them here: https://trello.status.atlassian.com/
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