Since a couple of days, I'm having a Tumblr blog. Can I use a FTP Client like Transmit or FileZilla to upload my HTML files etc.?
And if that is possible, can you tell me where I find those FTP login stuff?
Unfortunately not, tumblr requires you to upload any html code via their own system for updating your blog. You can however host things elsewhere, then refer/call them from your tumblr code.
People have tried small 'campaigns' before, to get tumblr to offer FTP but there hasn't been any visible success from these.