How is mime type of an uploaded file determined by browser?

Question

I have a web app where the user needs to upload a .zip file. On the server-side, I am checking the mime type of the uploaded file, to make sure it is application/x-zip-compressed or application/zip.

This worked fine for me on Firefox and IE. However, when a coworker tested it, it failed for him on Firefox (sent mime type was something like "application/octet-stream") but worked on Internet Explorer. Our setups seem to be identical: IE8, FF 3.5.1 with all add-ons disabled, Win XP SP3, WinRAR installed as native .zip file handler (not sure if that's relevant).

So my question is: How does the browser determine what mime type to send?

Please note: I know that the mime type is sent by the browser and, therefore, unreliable. I am just checking it as a convenience--mainly to give a more friendly error message than the ones you get by trying to open a non-zip file as a zip file, and to avoid loading the (presumably heavy) zip file libraries.

Answer 1

This is probably OS and possibly browser dependent, but on Windows, the MIME type for a given file extension can be found by looking in the registry under HKCR:

For example:

HKEY_CLASSES_ROOT.zip - ContentType

To go from MIME to file extension, you can look at the keys under

HKEY_CLASSES_ROOT\Mime\Database\Content Type

To get the default extension for a particular MIME type.