Hi there, in terms of can attachments be written to Portal or AGOL destinations? I believe a NEW packaged transformer is coming mid this year to assist here. You could try AttachmentWriter from the fme hub, a little old but edit it and adapt the transformers for your purposes. When reading existing AGOL feature with attachments, you need to expose the format attributes for “arcgisonline_attachment{}” list.

The type of workflow for writing, you will probably need to merge the FilePath to images using File and Directory path in a FeatureReader with the existing feature classes of the assets (based on objectID? or another ID). Ensure the image is moved into an attribute in the list ‘data’ using possibly AttributeFileWriter? When writing the asset, have the attachment format attributes ticked on and pass in the list? Also ensure attachments are enabled in the settings of the feature service.

I suspect you may encounter issues with existing Writer formats to Arcgis portal/online, hence the need for the packaged transformer coming this year.

Esri probably have python notebooks to upload attachments 

Hi, @plancet10.

This could be done in FME with either your original workspace or as a post-processing step. I would recommend doing it in a post-processing step.
There are two possible options. You could either use the custom transformer ArcGISEnterpriseAttachmentAdder or an HTTPCaller to call the REST API to upload the Attachment.

Either method will require you to have the features joined I believe, so that the Attachment will be added to the correct feature. 

Hope this helps!

I’d recommend you give this blog a read.  

https://community.esri.com/t5/arcgis-data-interoperability-blog/code-free-web-integration-with-a-touch-of-arcpy/ba-p/883683