Found the solution (I guess): The Download links expire after 2 hours (see https://support.airtable.com/docs/airtable-attachment-url-behavior?_gl=1*wob2oj*_ga*MTcxNzEwNDI0Mi4xNjc2NzEyNjE0*_ga_VJY8J9RFZM*MTY5MjI2ODcyMS42LjEuMTY5MjI2OTIwOC4wLjAuMA..)
Will now see that I don't send the URL, but the file to either mailchimp or a place that stores images longer...
Once you access the attachment via the URL, it will only be accessible at that URL for 2 hours.
So if you want access to an image or an attachment for more than 2 hours, you will need to move the image somewhere else. Since you’re using MailChimp, you can just manually upload the image to MailChimp when you are creating your campaign.
If you need this process to be automated, you can use Make to automate the process of moving the attachments out of Airtable and into a publicly-accessible cloud drive (such as Google Drive).
There is a small learning curve with Make, which is why I created this basic navigation video to help. I also provide the links to a few other Make training resources there as well.
p.s. If you have a budget for your project and you’d like to hire an expert Airtable consultant to help you with any of this, please feel free to contact me through my website: Airtable consulting — ScottWorld