You can then reference the file you uploaded in your comment, rather than doing this directly from comment itself (this is a workaround).
As for the error you see, this can be caused by a few things:
The exact cause of this issue could also just not be any of the above as this error is caused by various things and is still being reviewed by atlassian, see:
I will suggest also raising a ticket with Atlassian directly as then they could help you check the logs to ascertain what the actual cause is here.
Upload a file
1. There are two ways to attach a file to a page you are editing:
2. There are two ways to attach a file to a page you are viewing:
Regardless of the state of the page, you can upload multiple files at a time.
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