Antti,
1) When I try to log in with my developer account nothing happens, it just goes back to the sign in page asking me to put in my information,
2) I believe ArcGIS online attachments work different than the desktop attachments. When I had the trial version of the ArcGIS organizational account, I was able to upload a feature service through ArcGIS Desktop and once it was uploaded I could enable attachments in its properties in "My Content".
Thanks,
Tim