jquery-issue: cannot load the basic jquery

A plugin or your theme is probably calling wp_enqueue_media() and instead of adding a check for admin page and early return they are allowing the scripts to load on every admin page. You need to deactivate all the plugins and switch to the default theme. If this solves your issue then activate your plugins one … Read more

How can I demonstrate themes well?

Use a multisite installation. Let the main site be your announcement page and at least on sub-site the theme demo site. Then you can use special content like WP Test on that site. Some themes might require special content, like a shop theme that needs products. You can use another, separate sub-site for these themes.

Trouble creating custom sanitization function when uploading video files

validate_file doesn’t quite work the way you’re expecting it to – it checks if $file is in the allowed files array, not just its extension/mime type. You’re looking for wp_check_filetype. However, all that ultimately does is check the extension (i.e. it doesn’t read the headers of the file/ascertain the mime type otherwise). Unless you implement … Read more