The ATTX file extension seems to be used, or was used in the past for a special attribute file type for Cadence - now known as OrCAD.. Seems like it shares the same extension as netlist files.
How to open:
You should be able to import these files to Cadence.
How to convert:
As far as we know, this .attx file type can't be converted to any other file format. This is usually the case of system, configuration, temporary, or data files containing data exclusive to only one software and used for its own purposes. Also some proprietary or closed file formats cannot be converted to more common file types in order to protect the intellectual property of the developer, which is for example the case of some DRM-protected multimedia files.