More Accurate and Extensible MIME Type Handling

[mattonomics]

In the general case, a file extension may represent more than one MIME type. Currently, file extensions are married to a single MIME.

Specifically, let's look at the case of the "ico" file extension. Officially, the MIME type for this file should be "image/vnd.microsoft.icon". Unofficially, "image/x-icon" along with many others are usable.

The solution to this problem is extremely simple.

First, allow key => value pairs in the exhaustive MIME list located in wp_get_mime_types to be arrays so we can account for cases such as ico.

Second, add the proper handling for this change in wp_check_filetype. The way I have written it should not interfere with existing uses of this function (as they were all passing strings and now can pass a string or array).

[mattonomics]

Reference for the ico MIME type: ​http://www.iana.org/assignments/media-types/image/vnd.microsoft.icon

[mattonomics]

To really see the MIME issue in action, upload the favicon I've attached to this ticket while in multisite. Be sure to add ico to the list of accepted extensions!

[wonderboymusic]

Just closed #11824, we shouldn't bend over backwards just to support multiple mimes for favicons