This discussion is related to the FileUpload addon.
If you want guests to be able to download attachments from the File Upload plugin, you can make one small change to the database. I have not had any unwanted side effects with this so far. Here's the SQL to grants guests download permission:
UPDATE `YOUR_DATABASE_NAME_HERE`.`GDN_Permission` SET `Plugins.Attachments.Download.Allow` = '1' WHERE `GDN_Permission`.`PermissionID` =2;