This is a bit frustrating. Under Windows, the following happens:
When importing *.txt files from an import folder, the first line of the content is used as the note's title (as Frank.dg mentioned).
When importing *.html files from an import folder, the name of the file is used as the note's title, but with the extension.
I would like to import HTML files where I can define more precisely what the note's title should be. I don't want the file name with the extension. I was hoping that Evernote uses the content of the <title></title> tag or may be the first <h1>