An .asx file is typically a plain-text file that contains XML or HTML tags. A text file that contains any of the following tags is assigned a MIME type of "text/html" by Internet Explorer:
The MIME type that is detected by Internet Explorer for .asx files is frequently "text/html" because .asx files typically contain text and HTML tags such as <TITLE>.
Internet Explorer examines the actual contents that are associated with a local file to determine the MIME type. If the contents of the file do not permit Internet Explorer to determine the MIME type, the file name extension and associations or registered programs are used to determine the MIME type.