Restrict Media Library header image chooser to only images (no audio, video, etc)

[georgestephanis]

For 'Choose an image from your media library' users can select other media such as Audio or Video items

​http://cloud.stephanis.info/3l0z1g2q003T4530230o

The URL query string preselects the 'Image' file type, but others can be 'Set as header'

Setting an audio file as header obviously results in a broken image when cropping and a warning:

Warning: imagecreatefromstring() [function.imagecreatefromstring]: Data is not in a recognized format in wp-includes\media.php on line 258

Image could not be processed. Please go back and try again.

20819.2.patch​ also adds PHPDocs for the functions introduced in #19840.

We could also use one of our magic functions: __return_empty_array().

Replying to ocean90:

We could also use one of our magic functions: __return_empty_array().

Indeed, thanks.

Looks good.

In [20982]:

Don't show mime type filter links when choosing an image header from the media library. Props SergeyBiryukov, georgestephanis. fixes #20819

Replying to ocean90:

We could also use one of our magic functions: __return_empty_array().

Actually, no. That hides the file type links when calling the uploader window on Edit Post screen as well.

We need to check for context, like filter_upload_tabs() and attachment_fields_to_edit() do:
http://core.trac.wordpress.org/browser/trunk/wp-admin/custom-header.php?rev=20982#L969

20819.4.patch​ fixes the issue and brings the same changes to custom-background.php.

Can we simply do:

if ( isset( $_REQUEST['context'] ) && $_REQUEST['context'] == 'custom-background' )
    add_filter( 'media_upload_mime_type_links', '__return_empty_array' );

Yes, then we can remove the individual checks from attachment_fields_to_edit() and filter_upload_tabs() as well. Done in 20819.5.patch​

In [21009]:

Check context before adding media upload filters. Props SergeyBiryukov. fixes #20819