Make sure that your Acrobat or Reader preferences allow for the opening of attachments. The option is found at Edit > Preferences > Trust Manager > Allow opening of non-pdf file attachments with external applications.
What is the file type of your attachment? Acrobat has some security settings that don't allow you to open/extract some file types, like ZIP files. There is a way around it, but it requires a client side regedit.
I am running into a similar scenario where a flat file (ASCII) is attached to a PDF form and when the file has no extension, the file can be attached but can not be saved or viewed after the file is attached. Adobe gives the same error "Adobe Reader security settings do not allow this type of file to be opened."
We already have the preference set to allow the opening of non-pdf file attachments.
We have been able to edit the registry to allow the opening of a file with the .ASC extension, but when there is no extension, we are finding that Microsoft Registry settings will not accept the wildcard setting (.*).
We are running Adobe Reader version 9.1.
An alternate option:
you can attach the document as a Task Attachment (in the attachments tab) and later can make use of this attachment from the workspace.
I think the task attachment is more reliable and can open files of any kind.