The reason why you are being offered the upgrade could be the following: The file you are attempting to upload is larger than 5MB. Yola Free and Yola Bronze only allow you to upload individual files up to 5MB.
If you need to upload files larger than 5MB, you have the option to upgrade to a Yola Silver or Yola Gold account. Yola Silver allows you to upload files up to 100MB and gives you 5GB of total storage for your site while Yola Gold has a limit of 150MB per file with 10GB of storage per site. If you decide not to upgrade, you will need to select a file of 5MB or less.
For more information on Yola Silver, please see Yola Silver FAQ
For information on Yola Gold, please see Yola Gold FAQ.