I am not at my desk but I am fairly sure you are over writing the field when you do a record.update so you will need to get the file_id’s of the current files then push the new one into an array with the current and then add that array of id’s to your field.
Get current file_id’s
Buid an array with current and new id’s
Update record with the array of new and old ID’s
This would be the same if you you were doing a multi-select field as well.
Technically you should be able to merge the new file_id into the current array and extract into a new variable ready for the record update all in one JSONata evaluate but I didn’t think to do it that way when I wrote the above and I am not sure it is worth the brain power to work it out for one line.
Note the [] at the end of the evaluate string this forces JSONata to deliver an array even if there is only one item if you don’t have this in an there is a single file attachment the push will fail as it won’t be an array.