Came across another interesting bug today involving Firefox and mime types. Firefox uses a file called mimeTypes.rdf (stored in your profile folder) to keep track of a) what application should be opening the file you’re downloading and b) what kind of file it should tell a server it’s sending when you upload a file. And it works … for the most part. See, if you download a PDF file from a server that (incorrectly) states that the content-type of the file is ‘application/unknown’, choose to open it using Adobe Acrobat and then check the box that says ‘Do this automatically from now on’, Firefox will store that bit of knowledge away in mimeTypes.rdf. Now go and use a web application that you upload files to and which analyzes the content-type of the files you’re uploading and upload a PDF file. If you’re using LiveHTTPHeaders, you’ll notice that you’re not sending ‘application/pdf’ but instead ‘application/x-download’.
It looks like this bug was filed in bugzilla a couple times and even acknowledged in their documentation, but has yet to be fixed. You can ‘fix’ the problem by deleting your mimeTypes.rdf file and restarting Firefox.
My jaw dropped to the floor when I found out this very annoying bug is still there in 2011 (I mean your post is dated 2007). This is just ridiculous – In a webmail software, I try to upload a pdf file and it’s sent as text/html. The result: the recipient gets a lot of garbage text and no PDF. Checked out the file you mention, mimetypes.rdf, only to find PDF listed as an extension for text/html format.