It does not look for any extension, it just matches the mask. Unfortunatelly the default mask is wrogly defined. See "*.*htm*", which is meant to match both "*.htm" and "*.html". Unfortunatelly it matches even yours "xxxxxxx.html.zip". I'll change it in next release, change it yourself on your installation.

Hello,

My name is Erik and I have a bug to report.
When Preferences -> Transfer -> Transfer mode is set to Automatic and a file is being transfered with the name xxxxxxx.html.zip (which is evidently a zip file but also has the .html extension in the filename to mark that there are html files in this zip file) winSCP WILL transfer the file in 'text mode' in stead of the expected binairy that is supposed to happen with zip files. When the file is renamed to xxxxxxxx.zip it will transfer in binairy. It seems to me that winSCP looks at the first extension it finds in the filename.