Introduction
Wordpress comes out-of-the-box with hundreds of really well implemented features. It's one of the reasons it is the leading blogging software used today. One of the features we will focus on is the built in Wordpress Gallery.
To add a gallery to a post, you can follow the below steps:
Step 1: Click Add Media
Step 2: Click Create Gallery and select your photos
Step 3: Edit and Insert
If you have noticed though, you can only add Images. If you have uploaded videos to your Media, you cannot add them to a gallery.
The Fix:
The change we need to make is located within the function wp_ajax_query_attachments(). This function is located within the file, wp-admin/includes/ajax-actions.php. You can do a search for this function, or look at or around line 1835.
Note : The only way to easily support videos within a gallery through Wordpress's default media gallery is to edit a core file. That means that after an upgrade, you may lose this change.
Specifically, change line 1841:
's', 'order', 'orderby', 'posts_per_page', 'paged', 'post_mime_type',
To:
's', 'order', 'orderby', 'posts_per_page', 'paged',
Removing the 'post_mime_type' array index from this query will not add a constraint your media library to image/% mime types.
Summary
If you go back to a post, you can add or edit a gallery and now videos will display. By default, thumbnails are not created for your videos which you upload. I would suggest the Video Embed and Thumbnail Generator plugin to handle this.
