Here are some of the most common content types:
.htm, .html Response.ContentType = "text/HTML";
.txt Response.ContentType = "text/plain";
.doc, .rtf, .docx Response.ContentType = "Application/msword";
.xls, .xlsx Response.ContentType = "Application/x-msexcel";
.jpg, .jpeg Response.ContentType = "image/jpeg";
.gif Response.ContentType = "image/GIF";
.pdf Response.ContentType = "application/pdf";