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).

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

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!