In Addition to my topic with Acumatica REST API where I showed how to retrieve and save records using Acumatica Contract Base Rest API I want to share with you now how can you attach files with the same way.
To Attach file you just need to put binary data by following URL: https://<server>/entity/<endpoint>/<version>/<entity>/<key1>/<key2>/files/<filename>
For example, to attach file to stock item you will have following URL:
File will be attached to entity automatically.
Have a nice integration!