Issues with filemask

I'm having troubles getting the filemask to work properly. I hope you can see my error.

Using ListDirectory("/Build/TESTING"), I see this file/folder structure:

/Build/TESTING/.
/Build/TESTING/..
/Build/TESTING/.DS_Store
/Build/TESTING/A.rtf
/Build/TESTING/Alpha
/Build/TESTING/B.rtf
/Build/TESTING/Beta
/Build/TESTING/Omega

A.rtf and B.rtf are files
Alpha, Beta and Omega are folders

I'm trying to download B.rtf only. Here are examples of what I have tried.
options.FileMask = "B.rtf|*/"; //nothing copied
options.FileMask = "B.rtf | */"; //nothing copied
options.FileMask = "B.rtf; | */"; //nothing copied
options.FileMask = "B.rtf;|*/"; //nothing copied
options.FileMask = "B.rtf"; // B.rtf plus subfolders downloaded
options.FileMask = "|*/"; // nothing copied
options.FileMask = "| */"; // nothing copied
options.FileMask = "* | */"; // nothing copied
options.FileMask = "*.rtf | */"; // nothing copied
options.FileMask = ""; // all files and folders are copied

What is the correct filemask to only download B.rtf?

Thanks in advance.

If you want to download one specific file only, just name it in the GetFiles call:
See https://winscp.net/eng/docs/library_session_getfiles#remotepath

No FileMask is needed. Though the "B.rtf|*/" (and the like) should work too. If it does not, we need a session log file to investigate.