Work is currently in progress to improve the utility of the attachment cards in Jira. As part of these changes we will add a button to all attachment cards to directly download the attachment. We expect these changes to be delivered to you in the next month.
Note that in addition to improved attachment cards we are also actively working on bringing a list view to Jira's new issue view. This will provide an alternative way for you to find your attachments and will also include the ability to directly download an attachment: https://jira.atlassian.com/browse/JRACLOUD-70146
Using the old issue view, when you click in the attachment using the key Command the file is downloaded automatically, however, this feature is not available in the new issue view.
It's important because sometimes the user just needs to download the file and don't want to wait for the loading of the preview.
A button to download the single file under the thumbnail of the attachment would also help in this situation.
For big attachment files (over 8 MB) a Cloud icon is appears on the file allowing you to download it without having to preview the file. However, this is only applicable for Excel files.