By default, within the Postmark UI and via Messages API we will truncate the size of the returned message raw source (containing attachment data) to 1MB.
Within the UI, if you scroll down in the Raw Source tab when viewing the message you can see the attachment details, including the base64. For smaller attachments, the full base64 will be present and you from there you can save it to a file and then convert it back into the original file. You can do this by using a free service which allows you to decode base64.
If the base64 has been truncated, then you can instead use our API to get the full raw source containing full attachment data with the following query parameter: