how to use okhttp to upload a file?

Here is a basic function that uses okhttp to upload a file and some arbitrary field (it literally simulates a regular HTML form submission)

Change the mime type to match your file (here I am assuming .csv) or make it a parameter to the function if you are going to upload different file types

  public static Boolean uploadFile(String serverURL, File file) {
    try {

        RequestBody requestBody = new MultipartBody.Builder().setType(MultipartBody.FORM)
                .addFormDataPart("file", file.getName(),
                        RequestBody.create(MediaType.parse("text/csv"), file))
                .addFormDataPart("some-field", "some-value")
                .build();

        Request request = new Request.Builder()
                .url(serverURL)
                .post(requestBody)
                .build();

        client.newCall(request).enqueue(new Callback() {

            @Override
            public void onFailure(final Call call, final IOException e) {
                // Handle the error
            }

            @Override
            public void onResponse(final Call call, final Response response) throws IOException {
                if (!response.isSuccessful()) {
                    // Handle the error
                }
                // Upload successful
            }
        });

        return true;
    } catch (Exception ex) {
        // Handle the error
    }
    return false;
}

Note: because it is async call, the boolean return type does not indicate successful upload but only that the request was submitted to okhttp queue.