How the heck do I post a story?

Call me dense, but I'm having difficulty uploading a story.

I've got an .html file that contains my story. It has <p>, <br> and <i> tags and that's all. Well, maybe there's <html> at the start and </html> at the end.
I've used this same file to upload to tgfiction.net and it works OK. If I drag and drop the file into a Firefox window, it displays perfectly fine.
If I try and upload it here, what happens is that every <p>, <br> tag, plus every line feed in the source file gets turned into a paragraph break. This means that I end up with paragraphs with short lines with huge amounts of empty space between them. Elsewhere, linefeeds in my source files get discarded, and I naturally assumed that would happen with Topshelf.
I write using OpenOffice.org on Mandriva Linux. I normally write in Rich Text, and then when I'm happy export it to HTML and then load it with a text editor and clear out the junk that OO.o puts in before uploading. I'm assuming that the text editor is putting the linefeeds in, although I don't see why this should be a problem as they get tossed by Firefox and tgfiction.net.
Anybody got a clue what's happening, and how to deal with it?