To upload a file, one must send a specific type of HTTP request using the 'http' package.
A 'multiple request' must be created to send the file upload request.
In the HTTP client API of the app service, a method must be defined that returns a 'Future<HttpClientResponse>'.
This new method requires two named parameters: 'file' (marked with 'require' and of type 'PlatformFile') to indicate the file to be uploaded, and 'postID' (required) to signify the ID of the content to which the file belongs.
Prepare the file upload API endpoint using a URI parsed with 'Uri.parse', using the 'apiBaseUrl' from the 'appConfig', appending 'files' and a 'post' query with the value of 'postID'.
Create a 'HttpRequest' named 'request' as a 'MultipartRequest', with the HTTP method set to 'POST' and URL set to the prepared URI.
The request headers need to include an 'Authorization' header with a token for user authentication.
Add the file to be uploaded to the request as a 'MultipartFile' by creating it with 'MultipartFile.fromPath', including the file's path, and setting its 'ContentType' (use 'MediaType' with 'image' as the first parameter and a file extension, or default to 'jpeg' if not provided).
Import the package 'http_parser.dart' to use 'MediaType'.
Add the prepared 'MultipartFile' to 'request.files'.
