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Please read this guide: How to Increase Maximum Upload File Size in WordPress

The easiest way to figure out the problem is to request error logs from your hosting provider for your website, then examine them around the time when you tried to upload the file.
You will see what limit you have encountered and increasing it will solve the issue.

The settings in PHP that you likely need to adjust are:

• upload_max_filesize – set this to a value > than your backup
• post_max_size – set this to a value > than your backup
• memory_limit – set this to a value > than your backup
• max_execution_time – set this to 0 (infinite)

• This reply was modified 3 years, 5 months ago by Yani.

Adding some more info:

If you use nginx behind a proxy, you should also set the client_max_body_size variable to match the upload_max_filesize.