Hi blanca_luz, thanks for your message!
Maybe "doc" files work, I don't know, I will tell you what works for me.
My ebook file has an "html" extension.
Try this. Dump the following into a file called MyBeautifulToc.html, then create a new ebook, add MyBeautifulToc.html and follow the instructions.
<h1 class="mobi">Chapter 1</h1>
Hi, my name is TenaciousBadger and this is my first chapter.
<h1 class="mobi">Chapter 2</h1>
On to chapter two now.
<h1 class="mobi">Chapter 3</h1>
And chapter 3. Will we get a TOC?
Please let me know if you succeed!