This gives you the last date when the document that has an attachment was modified. It means I can change a document name but the attachment will be intact. You are getting document last modified date. Sorry this is not the correct answer.
Attachment versioning is more complex and depends how workflows are set up. Do you have versioning enabled in your system? There is a table called CMS_AttachmentHistory if you modified your attachment you must see the new record there (it doesnt tell who did though). There is a second table called CMS_VersionHistory this where you get a user who did modification. If versioning is enabled you can always "roleback" to previous version of your attachment. You must have Version tab in you document properties there you can see all document changes.
The other way is probably check the "staging" tables if you have the staging enabled, but not sure if you can get the user from there.