Upload binary file with okhttp from resources

You might not be able to do it directly using the library but you could create a little utility class which would do it for you. You could then simply re-use it everywhere you need it.

public class RequestBodyUtil {

    public static RequestBody create(final MediaType mediaType, final InputStream inputStream) {
        return new RequestBody() {
            @Override
            public MediaType contentType() {
                return mediaType;
            }

            @Override
            public long contentLength() {
                try {
                    return inputStream.available();
                } catch (IOException e) {
                    return 0;
                }
            }

            @Override
            public void writeTo(BufferedSink sink) throws IOException {
                Source source = null;
                try {
                    source = Okio.source(inputStream);
                    sink.writeAll(source);
                } finally {
                    Util.closeQuietly(source);
                }
            }
        };
    }
}

Then simply use it like so

OkHttpClient client = new OkHttpClient();

MediaType MEDIA_TYPE_MARKDOWN
        = MediaType.parse("text/x-markdown; charset=utf-8");

InputStream inputStream = getAssets().open("README.md");

RequestBody requestBody = RequestBodyUtil.create(MEDIA_TYPE_MARKDOWN, inputStream);
Request request = new Request.Builder()
        .url("https://api.github.com/markdown/raw")
        .post(requestBody)
        .build();

Response response = client.newCall(request).execute();
if (!response.isSuccessful())
    throw new IOException("Unexpected code " + response);

Log.d("POST", response.body().string());    

This example code was based on this code. Replace the Assets file name and the MediaType with your own.