Yeah, it’s really unfortunate that attachment fields aren’t editable in 2-way synced bases.

The workaround for this is that you will need to send your users outside of the destination base to edit the attachment field.

You would either need to send them to an interface page in the original base, or you could send them to a form in Fillout.

I think that the easiest & quickest workaround for this is to use Fillout’s advanced forms for Airtable.

Fillout is 100% free and offers hundreds of features that Airtable’s native forms don’t offer, including the ability to update Airtable records from a form.

So, you would create an “update record” form in Fillout that links to your original source base, and the form would contain the attachment field.

Then, back in your original source base, you would create a new formula field that Fillout provides for you. This will give you a unique record link for each record that will take you to your “update record form” in Fillout.

Optionally, you can create a button field that uses this formula.

Then, sync the button field (or the formula field) over to your destination sync table, and your users will have a button that they can click on the edit the attachment field in Fillout.

Hope this helps! If you’d like to hire an expert Airtable consultant to help you with anything Airtable-related, please feel free to contact me through my website: Airtable consultant — ScottWorld 

To Scott's point, unfortunately attachment fields aren't editable in 2-way synced bases yet with Airtable's native 2-way sync.

If you do need this though, you could use Whalesync's Airtable <> Airtable 2-way sync which supports Attachment field editing:

https://www.whalesync.com/connector/airtable