You will be prompted to download the HTM or HTML file (default document) if you browse http://servername or http://servername/vdir without specifying the file name in the URL. This occurs only if your default document is HTM and HTML files, not for ASP / ASPX
Fiddler trace shows that the request content type is an unknown MIME type and you will be prompted to download a document with no extension which is the default document of the website or the virtual directory.
Unknown MIME Type forcing the browser to download the document
Remove the wildcard MIME mapping or mapping for .HTM / .HTML associated with an unknown MIME type (e.g. Application/myMIME) in the Server level / Master level / File level MIME mappings.
To check the MIME mapping, go to the properties, click on HTTP Headers -> MIME Types.
In one of my case I worked today, the customer had a wildcard mapping as “application/octet-stream.” (Note the . in the MIME type).