Easier than using Postman would use curl direct on the terminal:
Here is the command that I use when necessary (I work with some services using loopback / explorer):
curl -i -X POST -H "Content-Type: multipart/form-data" -F " blob=@ /path/to/your/file.jpg" -v http://HOST:PORT/pathToYourEndpoint?access_token=xxxxxxxxxxx
source share