Sourtoe,
I can assure you that the files are .html files and are stored as .html files in the asset folder. What you describe to me looks like it could be either a configuration problem in your Windows folder display options or a problem with a default setting in your HTML editor. It does not look like a Trainz issue.
The files should be named filename.html with no ".txt" component in their names.
But firstly, what happens when you select one of the filename.txt.html or filename.html.txt files and rename it to filename.html ? Does that file now appear in the HTML Asset list in Surveyor? Also check that it is correctly named as filename.html in the config.txt file - although I have recently discovered that the filename entry, as it appears in the string table of the config.txt file, does NOT have to have the extension ".html" (it could be entered as just "filename") BUT the file itself must be an ".html" file.