This can be difficult to troubleshoot. But the fact that you see a 'contact your jira administrator' type error message is at least telling us that the Jira website is aware there is a problem with this upload. Which likely means there is an error in Jira's logs that will tell us more.
In this case, I would try to recreate the problem, and note the exact time you get this error message. Then let your Jira Admin known the exact date/time and the filename/jira issue key in question here. With that information, perhaps your Jira administrator can take a closer look into the logs Jira creates. These are stored in two different locations,
If Jira's logs have an ERROR level message that corresponds to the exact time you tried to upload this file, perhaps this can give us a clue as to why this problem is happening.
If that doesn't help, or you want to try another approach, You could use a browser like Chrome that has some native developer tools. Using these you can generate a HAR file that can capture an HTTP error messages that might be getting thrown to your browser, but might not be obvious to the end user.
Another general question, when you connect to this Jira site on the VPN, are you using the same URL/address of the site that you would use when at work? Or is the address different somehow?
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