refactor: 文件相关方法移动到helpers

src/platforms/h5/service/api/media/choose-image.js

-import { fileToUrl } from '../file/util'
+import { fileToUrl } from 'uni-platform/helpers/file'
 
 const {
   invokeCallbackHandler: invoke

src/platforms/h5/service/api/media/choose-video.js

-import { fileToUrl } from '../file/util'
+import { fileToUrl } from 'uni-platform/helpers/file'
 
 const {
   invokeCallbackHandler: invoke

src/platforms/h5/service/api/network/download-file.js

-import { fileToUrl } from '../file/util'
+import { fileToUrl } from 'uni-platform/helpers/file'
 /**
  * 下载任务
  */

src/platforms/h5/service/api/network/upload-file.js

-import { urlToFile } from '../file/util'
+import { urlToFile } from 'uni-platform/helpers/file'
 /**
  * 上传任务
  */