I was about to give up thinking that it wasn’t possible or at least easy and then I stumbled onto the wp_handle_upload_prefilter filter which gives you exactly what you asked for! Here’s the code:
add_filter('wp_handle_upload_prefilter', 'yoursite_wp_handle_upload_prefilter');
function yoursite_wp_handle_upload_prefilter($file) {
// This bit is for the flash uploader
if ($file['type']=='application/octet-stream' && isset($file['tmp_name'])) {
$file_size = getimagesize($file['tmp_name']);
if (isset($file_size['error']) && $file_size['error']!=0) {
$file['error'] = "Unexpected Error: {$file_size['error']}";
return $file;
} else {
$file['type'] = $file_size['mime'];
}
}
list($category,$type) = explode("https://wordpress.stackexchange.com/",$file['type']);
if ('image'!=$category || !in_array($type,array('jpg','jpeg','gif','png'))) {
$file['error'] = "Sorry, you can only upload a .GIF, a .JPG, or a .PNG image file.";
} else if ($post_id = (isset($_REQUEST['post_id']) ? $_REQUEST['post_id'] : false)) {
if (count(get_posts("post_type=attachment&post_parent={$post_id}"))>0)
$file['error'] = "Sorry, you cannot upload more than one (1) image.";
}
return $file;
}
And here are some screenshots showing how it looks in action:
