Create HTTP get response with attached binary data file and json formatted metadata

There's lots of ways to do this, I think the best choice is going to depend on what your receiving side is capable of understanding most easily.

The most literal interpretation would be to use content-type multipart/mixed https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html but you're probably going to have to write your own parsing on the receiving end. I don't know if this is exactly to spec, but it gets the idea across:

from http.server import BaseHTTPRequestHandler
import http.server
import socketserver
import string
import random
import io

PORT = 8000

class ResponsePart:
    def __init__(self, content, content_type):
        self.content = content.encode('utf-8')
        self.content_type = content_type.encode('utf-8')

class Mine(http.server.BaseHTTPRequestHandler):
    def get_separator(self, parts):
        while True:
            boundary = []
            for i in range(32):
                boundary.append(random.choice(string.digits + string.ascii_letters))
            boundary = ''.join(boundary).encode('ascii')
            for part in parts:
                if boundary in part:
                    break
            else:
                return boundary

    def do_GET(self):
        responses = [
                ResponsePart('abc123', 'Content-type: application/octet-stream'),
                ResponsePart('{"a":"b"}', 'Content-type: application/json'),
        ]
        boundary = self.get_separator([r.content for r in responses])
        self.send_response(200)
        self.send_header("Content-Type", 'multipart/mixed; boundary=' + boundary.decode('ascii'))
        self.end_headers()

        for piece in responses:
            self.wfile.write(b'--')
            self.wfile.write(boundary)
            self.wfile.write(b'\r\n')
            self.wfile.write(piece.content_type)
            self.wfile.write(b'\r\n')
            self.wfile.write(piece.content)
            self.wfile.write(b'\r\n')

Handler = Mine

with socketserver.TCPServer(("", PORT), Handler) as httpd:
    httpd.serve_forever()

With that out of the way, I'd do this probably using JSON or something so that you're returning a single consistent content-type:

from http.server import BaseHTTPRequestHandler
import http.server
import socketserver
import string
import random
import io
import json

PORT = 8000

class Mine(http.server.BaseHTTPRequestHandler):
    def do_GET(self):
        response = {
                'filedata': 'abc123',
                'status': {"a":"b"},
        }
        output_data = json.dumps(response).encode('utf-8')

        self.send_response(200)
        self.send_header("Content-Type", 'application/octet-stream')
        self.send_header("Content-Length", str(len(output_data)))
        self.end_headers()

        self.wfile.write(output_data)

Handler = Mine

with socketserver.TCPServer(("", PORT), Handler) as httpd:
    httpd.serve_forever()

This is going to be far easier to handle on the receiving end, one json decode and you're done.