// open folder method
    public void openFolder() {
        Intent intent = new Intent();
        intent.setType("*/*");
        intent.setAction(Intent.ACTION_GET_CONTENT);
        startActivityForResult(Intent.createChooser(intent, "Complete action using"), REQUEST_CHOOSE_DOCUMENTS);
    }

 @Override
 public void onActivityResult(int requestCode, int resultCode, Intent data) {
 switch (requestCode) {
 case REQUEST_CHOOSE_DOCUMENTS:
                if (resultCode == RESULT_OK) {
                    isAttachApply = true;

                    uri = data.getData();
try {
                                InputStream inputStream = getContext().getContentResolver().openInputStream(uri);

                                if (inputStream != null) {
                                    String name = fileName.substring(0, fileName.length() - 4);
                                    String suffix = fileName.substring(fileName.length() - 4);

                                    attachFile = new File(getContext().getCacheDir(), name + suffix);
                                    attachFile = copyInputStreamToFile(inputStream, attachFile);

                                    if (attachFile.length() == 0) {
                                        setWarningMessage(getString(R.string.apply_warning_attach_file_null_size));
                                    } else if (attachFile.length() >= MAX_FILE_SIZE) {
                                        setWarningMessage(getString(R.string.apply_warning_attach_file_big_size));
                                    } else {
                                        if (adapter.getItemCount() > 0) {
                                            adapter.setItemsNotActive(true);
                                            adapter.notifyDataSetChanged();
                                        }
                                        attachFileTitle.setTextColor(ContextCompat.getColor(getContext(), R.color.blue));
                                        warningAttachMessage.setVisibility(View.GONE);
                                        attachFileContainer.setBackgroundResource(R.drawable.blue_border_chosen_bg);
                                        isHasAttachWarning = false;
                                    }
                                }
                            } catch (FileNotFoundException e) {
                                e.printStackTrace();
                            }
                        }
                    }
                }

// create file inputstream
    private File copyInputStreamToFile(InputStream in, File file) {
        OutputStream out = null;

        try {
            out = new FileOutputStream(file);
            byte[] buf = new byte[1024];
            int len;

            while ((len = in.read(buf)) > 0) {
                out.write(buf, 0, len);
            }

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (out != null) {
                    out.close();
                }
                in.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return file;
    }
String mimeType = getContext().getContentResolver().getType(uri);
MediaType type = MediaType.parse(mimeType);
MultipartBody.Part body = MultipartBody.Part.createFormData("file", attachFile.getName(), RequestBody.create(type, attachFile));
Observable<Response<JsonObject>> observable = Api.get(getContext()).applyAttachRx(applyModel, body);

// interface
  @Multipart
  @POST("vacancy/applyattach")
  Observable<Response<JsonObject>> applyAttachRx(@Part("model") ApplyModel applyModel, @Part() MultipartBody.Part file);