Help Scout has a file size limit for attachments that are migrated over the API. The recommended solution is to edit the thread and remove the file attachment. You may opt to upload it to a service like Dropbox or Google Drive instead and use the given URL to add into the thread.
threads[10].attachments[0].data - Attachment is too large, it must be less than 10 MB

Written by Ashley Milne
Updated over a year ago