post https://go.v7labs.com/api/workspaces//projects//entities//properties//start_file_upload
Start the file upload process for a given entity property.
Use the retrieved file_upload_url to upload the file to the storage.
After file is uploaded use returned confirm_upload_url to confirm the upload.
Be sure to use provided confirm URL as this request may redirect you to different
Sub-Project as result of upload initiation.
Callback