I have about 3000 points with a small PDF attachment for each point (200 to 300kb attachment size). When I publish to ArcGIS.com from ArcMap Desktop 10.4, it takes a long time, shows the attachments copying up to ArcGIS.com. However when I view the feature service in ArcGIS.com Pop-up, there is no attachment.
There is a know issue with this, but it relates to an "ObjectID" field being renamed, but in this case, my was not and still reads "ObjectID" http://support.esri.com/en/technical-article/000011888
Any thoughts or workarounds? I recreated the entire layer and re-published it a second time, but the attachments still disappear in ArcGIS.com