Hey Bjarne,
since you're running on cloud, I would suggest to contact the Atlassian support first.
If you were hosting yourself, a look in the JIRA log should provide some additional information to work with, but this way, it could be anything really.
Meanwhile, you could use Firebug or other developer tools in your browser to look for issues during the upload or test with another browser to see if it's working with this.
In case you're accessing JIRA through a proxy, it could also be a limitation within the proxy (filetype, filesize, etc.) that interrupts the upload.
Regards,
Philipp
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