The order of the files in an attachment field can be manually reordered or reversed with a toggle.
Reverse order toggle
An attachment field may have a toggle "Show attachments in reverse order" in your base if it was created before April 2020. This toggle allows you to change the direction of the existing files in the cell and also affects the order of newly added files.
When the toggle is on (legacy behavior), any new file you attach will be added to the beginning of the list. The default for new attachment fields is for the toggle to be off so that files are listed in the order they are added to the cell.
This option to toggle this legacy behavior will only be available on older bases created when this was the default such as in many of the templates in our template gallery and Airtable Universe. Once you untoggle this option on all attachment and linked record fields in the base, the option to show the files in reverse order will no longer display.
To reorder attachments, expand one of the cells in the attachment field. Drag and drop an attachment to establish a different order.
You can also reorder attachments in an expanded record, or in attachment preview.