Improvements: file extensions & contents

Thanks for this pretty nice plugin, I have a few suggestions for you based on some usage.

1. I’d replace line 370 file_get_contents with wp_remote_fopen this will help w/ servers that don’t allow for allow_url_fopen open to be set.
2. On line 480 you’re limiting the file extensions to 3 characters preg_match_all('/'.$base_uploads_url.'\/.*?\.[a-z]{3}+/i', $attachment, $attachments); the problem with this is that it excludes document types like docx, pptx, xlsx and jpeg which will invalidate the verify_exists() check you have running on line 492 because pptx will then become ppt. So if just replace at a minimum the {3} with a {3,4} things should be golden again.
3. Allow us to extend get_available_content_types() by applying it to a filter/hook. That way we can add other MIME types that you don’t currently have in the list.

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