c# - Creating a file with an alternative data stream in MockFileSystem

Question

I'm writing some code that unblocks files that have the Zone.Identifier stream on them. The code works but I can't seem to get a unit test to work to test it. In the test I want to create a file which has the alternative data stream on it in a MockFileSystem. The problem is that the System.IO.Abstractions.TestingHelpers.MockFileSystem class doesn't support colons in the paths. This is the code I'm using:

public void AlternateDataStreamExistsTest()
        {
            var fileSystem = new MockFileSystem();

            using (MemoryStream ms = new MemoryStream())
            {
                using (Stream fs = fileSystem.File.Create(@"a.txt:Zone:Identifier"))
                {
                    using (var streamWriter = new StreamWriter(fs))
                    {
                        streamWriter.WriteLine("[ZoneTransfer]");
                        streamWriter.WriteLine("ZoneId=3");
                    }
                }
            
            }

            FileUnblocker fileUnblocker = new FileUnblocker(fileSystem);

            bool result = fileUnblocker.AlternateDataStreamExists(@"a.txt", "Zone.Identifier");

            Assert.IsTrue(result);
        }

Test method threw exception: System.NotSupportedException: The path is not of a legal form.

This is the error I get in return. I know the package Trinet.Core.IO.NTFS exists, but I can't use that for undisclosed reasons.