Do you support .SRT files?

At this point, the question becomes: what exactly should you upload? 

Publitio supports .VTT file upload and is easily allowing you to connect your .VTT captions with your video files.

If you have a .SRT caption, you can use a subtitle converter and convert .SRT caption to .VTT caption free. https://www.happyscribe.com/subtitle-tools/convert-srt-to-vtt