Change content type detection header size to 4k

There is a problem detecting some file types like SVG. Depending on how the file is built, it will detect the right content type or not. For example, if the SVG file has a big html comment (more than 512 bytes) at the beginning of it, when we start the detection process we'll get the first 512 bytes and based on that we will set the content type text/plain. But this is wrong because the file is indeed an SVG.