With the upgrade to New York, a new field "hash" is added on the sys_attachement table. This is to uniquely identify the content of an attachment. A hash is a 64-character identification number and it is assigned to the attachments. However, this value is calculated for the newly created records since New York upgrade only. Unfortunately, there is no way to manually generate the hash value for the existing attachments as it is calculated from the platforms inbuilt java code.
Refer to this doc for more details about the new features in the New York release.