We upload our videos via FTP. How do we upload the subtitle files belonging to these videos?

Subtitle files for videos uploaded via FTP can be uploaded on the video editing page of each video inside your Lightcast.com account once the transcoding process is completed. A subtitle file has to be connected with a video file and they are uploaded within the metadata of the video file in order to establish an unmistakable connection to the corresponding video. 

Login to your Lightcast.com account and go to "Video Archive". For opening the editing page, click on the video file you want to upload the .srt file for and switch to "Metadata".  Within the "Metadata" tab, scroll down to the .srt file uploader at the bottom of the page to upload your .srt file.