Force lower case.

I want to always convert files names to lower case on the remote site. Cannot find how to set this preference n Options.

Go to Transfer page in Preferences:
https://winscp.net/eng/docs/ui_pref_transfer
And in the Defaults, set Filename modification > Lowercase:
https://winscp.net/eng/docs/ui_transfer_custom

Thanks for the reply. It did not look like Default had a submenu.

It was certainly invisible to me, and when I asked for help, I was directed to "Transfer Settings" rather than "Preferences," and there there is no such option in the sub-menu!
Very annoying; ought to be fixed.
Thank you for asking!

@slowreader: In your thread, until you posted a screenshot, it was not not clear to me that you were referring to the Transfer Settings presets toolbar. My instructions were for the Transfer options dialog.