This video explains how to retrieve attachments from DEVOPS work item using API.
Expressions and API endpoint I used are:
/_apis/wit/workitems/{replace here your work item id}?$expand=all&api-version=7.1
last(split(outputs('Compose'),'/'))
_apis/wit/attachments/{replace here attachment id}?api-version=7.1
outputs('Attachment_Details')?['$content']
base64ToBinary(outputs('Attachment_Content'))
Microsoft Documentation:
https://learn.microsoft.com/en-us/rest/api/azure/devops/wit/attachments/get?view=azure-devops-rest-7.1&tabs=HTTP