This is a known issue of IE since - at least - 3 years...
found the snippet below in my textfile archive - must be from early 2004 (no source link, sorry):
If you aren’t familiar with this misfeature, well, it’s a “everyone’s at fault” thing where Apache serves up files with unknown extensions with a default MIME type, rather than with no MIME type at all, so Internet Explorer only rarely believes the server-provided MIME type, and instead looks at the content and decides what it really is.
It certainly
isn’t an “everyone’s at fault” thing. From RFC 2616 (the HTTP 1.1 specification):
If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource.
Basically, any attempt at file type guessing when a Content-Type header is supplied is a flat out violation of the specification. The blame
entirely lies with Internet Explorer. "