This probably applies to all platforms, but I can't test it. A file with periods in it, for example, a.b.html, is not correctly recognized as an html file (at least using a file:// URI). I suspect there is a DOS-ism in Mozilla which assumes everything after the first period is the file extension, so it looks for ``b.html'' and can't find a MIME type for it. What you need to do is look for endings, so you check ``does it end with .html? yes? then it is text/html'' -- in which case you have to check longer extensions (such as .tar.gz) before shorter ones (.gz).
Guessing that this falls under URL resolution...
*** This bug has been marked as a duplicate of 21789 ***
verified duplicate of 21789.