Specifying tab delimiter does not work

I'm trying to upload a tab-separated import file. I've tried every combination to specify tab separators I can think of:

• /t and '/t' - as suggested on import page (bit odd because I'd expect to use backslash for tabs, that is: \t)
• \t and '\t'

with no luck. The next screen always offers me one column with all my header names.

Am I missing something? Or is this in fact a bug?

Also, I'm not sure why there is also a "File Delimiter" option on that next screen. Doesn't it need to be already specified in order to display the columns?