According to the Internet Assigned Numbers Authority (IANA), all .ico file falls under the MIME type image/vnd.microsoft.icon. (Source)
E.g.
<link rel="icon" type="image/vnd.microsoft.icon" href="favicon.ico" />
However, savvy internet guru, Paul Irish, claims this is wrong, and that it would actually be image/x-icon. (Source)
E.g.
<link rel="icon" type="image/x-icon" href="favicon.ico" />
I know you can get away with not including a "type" for .ico files, but if you were going to include one, which should it be? Are there actually any problems with serving it as official IANA type?
When you're serving an .ico file to be used as a favicon, it doesn't matter. All major browsers recognize both mime types correctly. So you could put:
<!-- IE -->
<link rel="shortcut icon" type="image/x-icon" href="favicon.ico" />
<!-- other browsers -->
<link rel="icon" type="image/x-icon" href="favicon.ico" />
or the same with image/vnd.microsoft.icon, and it will work with all browsers.
Note: There is no IANA specification for the MIME-type image/x-icon, so it does appear that it is a little more unofficial than image/vnd.microsoft.icon.
The only case in which there is a difference is if you were trying to use an .ico file in an <img> tag (which is pretty unusual).
Based on previous testing, some browsers would only display .ico files as images when they were served with the MIME-type image/x-icon. More recent tests show: Chromium, Firefox and Edge are fine with both content types, IE11 is not. If you can, just avoid using ico files as images, use png.