I just wanted to add my support for this idea – I think it makes a lot of sense!
My suggestion would be to enable it via a setting (but probably have it enabled by default).
Settings->Download Attachments
Automatically include new uploads in Download Attachments ( yes | no )
Then, when a user uploads an attachment while creating/editing a Post or Page, it will also get added to the Download Attachments metabox, but could still be removed manually if desired.
However, I think this functionality might be lacking something important: the ability to filter which types of files get added as attachments.
For example, it makes a lot of sense that PDFs, Excel files, mp3s, etc should be available as attachments to download. However, I’d think that things like images or logos used within the post would very likely NOT want to be listed here. This could be addressed as follows:
Automatically include new uploads in Download Attachments ( all files | all files except images | no files )
or a text box where users could enter extensions to include/exclude as file types that get added automatically. For example, a user might choose to automatically add files that are NOT in the following list: .jpg,.jpeg,.png,.gif,.bmp
Cheers