Uploading binary assets such as images requires sending the data as a file stream. When using the Python requests library, the files parameter handles the construction of multipart/form-data automatically. The dictionary keys correspond to the form field names expected by the API, while the values specify the file metadata and content.

import requests
import os

def transmit_image_resource(directory, filename, mime_type='image/jpeg'):
    api_endpoint = 'https://api.service.example/upload'
    full_file_path = os.path.join(directory, filename)

    # Defining the multipart payload
    # Fields expecting files use a tuple: (filename, file_object, content_type)
    # Fields expecting simple data use a tuple: (None, value)
    form_data = {
        'source_id': (None, 'sensor_unit_01'),
        'captured_image': (filename, open(full_file_path, 'rb'), mime_type),
        'capture_time': (None, '2023-11-15 14:30:00')
    }

    # Execute the request using the 'files' parameter
    # The context manager ensures the file handle is closed properly
    with open(full_file_path, 'rb') as file_handle:
        form_data['captured_image'] = (filename, file_handle, mime_type)
        response = requests.post(url=api_endpoint, files=form_data)
            
    return response.()

For scenarios requiring manual control over the multipart/form-data encoding or when integrating with libraries that need pre-encoded bodies, urllib3 can be used. This method involves reading the file content into memory, defining the fields, and generating the raw byte string and the corresponding Content-Type header.

from urllib3 import encode_multipart_formdata
import requests

def upload_text_document(file_location, access_token):
    target_url = "https://api.service.example/docs/upload"

    # Read file content as text
    with open(file_location, mode="r", encoding="utf-8") as f_obj:
        content_data = f_obj.read()

    # Construct the field dictionary for encoding
    multipart_fields = {
        "document_file": (os.path.basename(file_location), content_data),
        "category": "reports"
    }

    # Manually encode the data to get the body and Content-Type header
    encoded_body, content_type_header = encode_multipart_formdata(multipart_fields)

    # Set up headers including the dynamically generated Content-Type
    request_headers = {
        "Content-Type": content_type_header,
        "Authorization": f"Bearer {access_token}"
    }

    # Send the request using the encoded body in the 'data' parameter
    server_response = requests.post(url=target_url, data=encoded_body, headers=request_headers)
    return server_response.()