Maximum file upload size : Maximum allowed file size when users upload attachment or thumbnail files.
Note: Most PHP installations have this maximum set to 2 MB (php_ini file).
Attachment file types : Comma-separated list with valid attachment file types that can be uploaded (e.g. \'.pdf,.jpg,.gif,.png,.mp4,.avi\').
Thumbnail file types : Comma-separated list with valid thumbnail file types that can be uploaded (e.g. \'.jpg,.jpeg,.gif,.png\').
Thumbnail - maximum size : Maximum thumbnail image size. If users upload larger thumbnails, the thumbnails will be automatically resized to the maximum size.
Note: High thumbnails will stretch the day cells in Month view, which may result in undesired effects.
Thumbnail delete margin : If a thumbnail is used since this number of days ago, it can not be deleted.
The value 0 days means the thumbnail can not be deleted.