Data Api Uploaded image is broken

I’m uploading image to Bubble API backend from our Angular front-end app. Server returns 200 status, but image is white square, so there are defenitely some problems wit upload.
Algorithm i use:

  1. User selects a file
  2. Application extracts file contents with fileReader.readAsBinaryString()
  3. Application encodes data as Base64
  4. Data is sent to the api endpoint

Request is processed and image is saved, because previous one gets deleted.
For request paylod i followed this instruction https://bubble.io/reference#API.sending_data.

What is right way to send image to server?