How to validate file uploads by mime type

I've built my first file upload script, and I'm needing to be able to validate the files by their mime type, but I'm running into some problems that I can't find answers to via Google or here. Here's an example of the code I'm using so far:

//Check extensions for documents
if ($resource_type == 'document') {
$allowed = array('application/pdf' => 'pdf',
'text/rtf' => 'rtf',
'application/rtf' => 'rtf',
'application/msword' => 'doc',
'application/octet-stream' => 'doc',
'application/vnd.ms-excel' => 'xls',
'application/vnd.ms-publisher' => 'pub',
'application/ppt' => 'ppt',
'application/vnd.ms-powerpoint' => 'ppt',
'text/txt' => 'txt',
'text/plain asc ' => 'txt');

//Check that the uploaded type is allowed.
if (!array_key_exists($_FILES['resource']['type'], $allowed)) {
$Error_Stat = 1;
$Message = Error("That file type is not allowed for documents.");
}
}

So basically, it's looking at the type and making sure it is of certain kinds that I specify. It's working great for most files, but one .doc file I uploaded was the type "application/octet-stream." What is that? I know it should be "application/msword," but why is it different?

I think its a fall back? If it can't determine what it is, it falls back to that identifier?

I do know its for forcing downloads for any file. Not sure on the upload part though? Can anyone else chime in on this?

iLLin is correct about it being a fallback. It is kind of like saying, "this is a file consisting of bytes" (octet - 8 - 8 bits - byte).

Not sure why one specific word document would result in that MIME type. Look at the header() documentation on php.net. There's a lot of discussion on MIME types there.

I do know its for forcing downloads for any file.

This is what the MIME type 'application/force-download' is for. :)

Is there a better way to do mime type validation, or at least make sure that only certain files get through (.doc, .rtf, etc. for document category - .mpg, .mov, etc. for video category - etc.)?

There isn't much you can do.

I just check the extension. Most uploads I have are for internal (intranet) purposes, so the security risk is not as big.

You get a collection of bytes. The only information you have about it is the file name. The contents cannot be trusted.