There is a vertically aligned rectangle. The left and right sides are longer than the top/bottom sides. In the top right corner there is a small right triangle built into the rectangle, creating a diagonal line in place of part of the top and right sides. In the middle of the rectangle are the letters 'HTML'
This is a HTML Filetype icon. It is available in PNG and vector. Downloading is free for PNGs up to 100x100px.