Retrieves a list of blacklisted attachment types.

An attachment blacklist is a list of file types that a signer cannot upload to a transaction.
For example, .exe files may be blacklisted, meaning that a signer cannot attach an .exe file
to the transaction.

Returns a list of blocked file extensions (e.g., "EXE", "DLL", "MSI", "DMG", "SO").

Note: This endpoint is READ-ONLY. The blacklist is managed by OneSpan system administrators
and cannot be modified via the API.