The d81 file extension is mainly related to a special emulator format that mimics the VC-1581 diskette for Commodore computer.
These diskettes were originally used on C64 and C128 computers.
How to open:
You can open these files using emulators like VICE.
How to convert:
As far as we know, this .d81 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.