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You generally open a given file type by running the application that created the file. On Windows, essentially any file can be opened with Notepad. However, "opening" a file usually isn't particularly useful. Instead, you need something that can "process" the file.
A file extension, e.g., ".ABS", might or might not be useful. Anyone can put any extension on any file. The extension doesn't determine the data format, nor does the format determine the extension. Extensions are regularly used by multiple programs for different purposes. If you don't know what program to use, it's guesswork.
If it's your file, you should be the one who knows how to process it. If someone sent the file to you, you need to ask the sender. If you don't know what the file is and it was sent to you, you shouldn't be "opening" it -- you should delete it.