It is assumed that a mime type has a minimum of two parts when splitted by "/"

[Chouby]

While testing the version 3.5-beta3-22734, I got the following notice (due to an attachment created without media file):
PHP Notice: Undefined offset: 1 in /wp-includes/media.php on line 1242
The line is:

list( $type, $subtype ) = explode( '/', $attachment->post_mime_type );

See also: #22186

[nacin]

Yeah, we should avoid making this assumption.

[ryan]

In 22830:

Handle attachments that have an empty mime type or a mime type without a subtype.

Props SergeyBiryukov
fixes #22532

[ryan]

​[UT1156]