We have a feature service with attachments enable that was published through ArcGIS server/Portal 10.7.1 which we are accessing through AGOL with the rest service url and stored credentials. We can add attachments using the service in ArcMap and through the Portal in a webmap, but not in AGOL. Is there a restriction in adding attachments while accessing a service through AGOL with stored credentials? We get the error that the "file type is not supported" even though all files types are supported in Portal and ArcMap. We have tried to just use the rest end point with no credentials stored in AGOL, but it seems that we would have to login every time which is something we would not like to do, but adding attachments do work in a webmap in AGOL. So is there a way to store the credentials and still add attachments to our on premise published feature service in AGOL? I'm not sure what to do.