The VTT (WebVTT) subtitle file to upload. Only .vtt format is supported.
Maximum file size: 1GB. Note: For testing in the API playground, the upload
limit is 5MB.
The source language of the content in the uploaded file. Use one of the codes
listed in Supported Languages.
The special value all is also accepted for auto-detection workflows.
Unique identifier for the project associated with the uploaded VTT file. This
confirms the successful association of the subtitle file with the project.