Please check as described here: https://confluence.atlassian.com/doc/configuring-attachment-size-150040.html the file soze limit and adjust if it is less than 20MB.
Note however that you can have a proxy limitation on attachments in which case your server admin (not Confluence admin) would have to make changes to this.
You can know that the problem is on the server by opening the browser developer console and checking where the timeout is coming from.
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