I have this:
<textarea name="content"></textarea>
and in javascript code in the same aspx file I call init to tinyMce.
I have html file which is in the same folder as this aspx file. How to make it show in the tinyMce?
Eighter render the content of your html file into the textare directly OR use ajax to get the file contents and set them as content of your editor. I assume your tinymce editor is already initialized then you may set the content from the ajax response using
tinymce.get('my_editorid').setContent(html_content);