解决方法
我或多或少地从一些预生产代码中复制了这个：
OkHttpClient.Builder builder = new OkHttpClient.Builder().proxy(proxy).writeTimeout(10,TimeUnit.MINUTES);
this.client = builder.build();
// ...
Request.Builder requestBuilder = new Request.Builder().url(uploadUrl);
for (TagEntry entry : item.getUploadUrlHeaders()) {
requestBuilder.addHeader(entry.getKey(),entry.getValue());
}
RequestBody requestBody = RequestBody.create(BINARY_MEDIA_TYPE,file.toFile());
requestBuilder.put(requestBody);
final Response response = client.newCall(requestBuilder.build()).execute();
assert response.isSuccessful();
