POST https://api.ebay.com/post-order/v2/return/5********1/file/upload
{
  "fileName" : "5********1_s***********l.jpeg",
  "data" : "SGVyZSBpcyB5b3VyIHJld...YWNraW5nIG51bWJlciBpcyAxMjM0NTY3ODk4NzY1",
  "filePurpose" : "LABEL_RELATED"
}