The first step will be to convert the blob to base64

After, you have two options
A) send directly the file to Bubble /fileupload endpoint and after, send the url returned by this request to your actual backend request
B) send a JSON instead of a multipart-form with something like

{
    "file": {
        "filename": "filename",
        "contents": "Base64-encoded binary data"
    }
}