This is the only thread that I have come across that addresses this issue, but I am experiencing the same behavior in ArcGIS 10.1. Just like the original poster, I have tried:
1. GP tool and python via python window in Desktop.
2. Tried running GP tool in an edit session.
3. File(s) exist on disk on the machine that I am running this tool from.
4. Tried mapped drive and local paths.
5. I can add an attachment manually, so that is working and I have the necessary rights to do so.
6. Tried running GP tool from Catalog and Desktop
7. Etc.
The error is the same as well: "Cannot add file 'X' to feature with OID = 'X'"
The feature class participates in a replica, so making a copy is not an option.
The environment is ArcGIS for Desktop Standard 10.1, SP1. Database in SDE; geodatabase version matches the ArcGIS release.
Thanks!
Tim