Why do fstream.read and fstream.write use char and not unsigned char?

As far as I know, read() and write() are there so we can read and write bytes directly from or to a file, and I was taught that the equivalent of a byte in c++ is unsigned char, so why do they take char pointers as parameters?

Also, do take a look at this function from a "bmp file image reader" library I found:

bool BMPImage::readInfo()
{
    //...

    //read bmp and dib headers
    unsigned char header[28] = {0};
    _ifs->read((char*)header, 28);
    _width    = *(int*)&header[18]; //width is located in [18] and is 4 bytes size
    _height   = *(int*)&header[22]; //height is located in [22] and is 4 bytes size
    _bpp      = (unsigned char) *(short*)&header[28]; //bpp is located in [28] and is 2 bytes size
    _channels = _bpp / 8; //set num channels manually

    //...

Why does the _ifs->read() line work anyway? The cast from unsigned char to char forces loss of data, no?