Despite issue marked as solved, I would like to clarify some details, it may help to somebody in future.
Nope, it’s possible, but needs additional efforts
According to reference:
When updating an existing attachment cell value, the specified array will overwrite the current cell value. If you want to add a new attachment without deleting the current attachments, you can spread the current cell value…
To conclude:
- attachment cell value is always array of attachments (e.g. [ x ] = x1,x2,x3…)
- often that array includes single element X, but that’s still array
- if you overwrite it with another field value, (array of Y), it’s OK
- if you want to “extend the field’s content”, you need to add previous + new value and put into cell and here is the problem:
table expects single array, like [x1,x2,x3,y1,y2…]
Automation takes value from step1, adds values from step2 and puts it into new array to put in field.
Why test doesn’t work then??
if you press on A_Import, you should see something like:

I think, automation counts that “orphaned comma” as “unexpected end of array” and refuse to create it.
so, if automation runs on empty attachment field, or if it runs with zero “find record” result, it may fail.
if it’s okay, then it’s done
Otherwise, you can insert (after Find records) script step which takes step 1 and 2 results, and
if value 1 is empty, result = value 2
if value 2 is empty, result = value 1
if both are empty, result is empty
if both are not empty, result=value1+value2 (cocatenate 2 arrays)
in short, it may be smth like:
And then you may create next step “Update record” and use that value, “result”
P.S. later, you may encounter ‘duplicates issue’ when each automation run adds content and doesn’t check if it already exist in result field. Also, you can’t rely on ‘concat removing duplicates’ because similar attachemt in different field has different url.
Then you can turn it to smth like my script at the beginning 