I think you have to set the correct MIME Type. It is a bit hidden. In the Multipart Upload, click in the Value field, then click on the ... button.

Thanks @nielsgerrits​ ,

After clicking Edit Parameters for the value column I've set mime to: image/bmp but attachment file not found error remains.

E.g. R:\\Libraries\\Images\\SE photos non spatial\\Gates\\stricklandgate10.bmp is a valid path in my DocumentLink field and returns an image but still returning the error.

I do not have a working sample lying around for uploading attachments, but I have something similar (update thumbnail) and I put the token and the format in the multipart form upload, not in the query string parameters. Not sure if that is your problem, but it's worth a try?

If I check the documentation, it expects a different input parameter? Not "fileobj" but "attachment"

Thanks @nielsgerrits.

I needed thumbnails to show in the pop up gallery of an AGOL web map, instead of a download link.  I was using the Arcade expressions workaround and not too happy with that.

The not so obvious mime-type parameter did it for me. 

I suggested an idea to improve the HTTPCaller and also address multiple mime-type attachments. Please upvote if you like  this idea.