Encoding the multipart/form-data file in Apex. Extract from https://github.com/muenzpraeger/salesforce-einstein-vision-apex/blob/master/src/classes/EinsteinVision_HttpBodyPart.cls

EinsteinVision_HttpBodyPart.cls

public virtual class EinsteinVision_HttpBodyPart {
    
    //...
    
    /**
     *  Write a key-value pair to the form's body for a blob.
     */
    public static string WriteBlobBodyParameter(string key, string file64, string filename, string mimeType) {
        string contentDisposition = 'Content-Disposition: form-data; name="' + key + '"; filename="'+filename+'"';
        string contentDispositionCrLf = contentDisposition + '\r\n';
        blob contentDispositionCrLfBlob = blob.valueOf(contentDispositionCrLf);
        string contentDispositionCrLf64 = EncodingUtil.base64Encode(contentDispositionCrLfBlob);
        string content = SafelyPad(contentDisposition, contentDispositionCrLf64, '\r\n');
        
        string contentTypeHeader = 'Content-Type: ' + mimeType;
        string contentTypeCrLf = contentTypeHeader + '\r\n\r\n';
        blob contentTypeCrLfBlob = blob.valueOf(contentTypeCrLf);
        string contentTypeCrLf64 = EncodingUtil.base64Encode(contentTypeCrLfBlob);
        content += SafelyPad(contentTypeHeader, contentTypeCrLf64, '\r\n\r\n');
        
        integer file64Length = file64.length();
        String last4Bytes = file64.substring(file64.length()-4,file64.length());

        // Avoid padding the file data with spaces, which SafelyPad does
        // http://salesforce.stackexchange.com/a/33326/102
        EndingType ending = EndingType.None;
        if (last4Bytes.endsWith('==')) {
            last4Bytes = last4Bytes.substring(0,2) + '0K';
            file64 = file64.substring(0,file64.length()-4) + last4Bytes;
            ending = EndingType.CrLf;
        } else if (last4Bytes.endsWith('=')) {
            last4Bytes = last4Bytes.substring(0,3) + 'N';
            file64 = file64.substring(0,file64.length()-4) + last4Bytes;
            ending = EndingType.Cr;
        }
               
        content += file64;
        content += WriteBoundary(ending);
        return content;
    }

    /**
     *  Helper enum indicating how a file's base64 padding was replaced.
     */
    public enum EndingType {
        Cr,
        CrLf,
        None
    }
}