新人想请教egg-multipart问题：使用File 模式处理文件上传
问题现象：批量上传文件时会出现某几个文件始终处于上传状态，对应的上传请求处于挂起状态。
BUG排查：
```js
// node_modules/egg-multipart/app/extend/context.js
async saveRequestFiles(options) {
    options = options || {};
    const ctx = this;

    const multipartOptions = {
      autoFields: false,
    };
    if (options.defCharset) multipartOptions.defCharset = options.defCharset;
    if (options.limits) multipartOptions.limits = options.limits;
    if (options.checkFile) multipartOptions.checkFile = options.checkFile;

    let storedir;

    const requestBody = {};
    const requestFiles = [];

    const parts = ctx.multipart(multipartOptions);
    let part;
    do {
      try {
        part = await parts();
      } catch (err) {
        await ctx.cleanupRequestFiles(requestFiles);
        throw err;
      }

      if (!part) break;

      if (part.length) {
        ctx.coreLogger.debug('[egg-multipart:storeMultipart] handle value part: %j', part);
        const fieldnameTruncated = part[2];
        const valueTruncated = part[3];
        if (valueTruncated) {
          await ctx.cleanupRequestFiles(requestFiles);
          limit('Request_fieldSize_limit', 'Reach fieldSize limit');
        }
        if (fieldnameTruncated) {
          await ctx.cleanupRequestFiles(requestFiles);
          limit('Request_fieldNameSize_limit', 'Reach fieldNameSize limit');
        }

        // arrays are busboy fields
        requestBody[part[0]] = part[1];
        continue;
      }

      // otherwise, it's a stream
      const meta = {
        field: part.fieldname,
        filename: part.filename,
        encoding: part.encoding,
        mime: part.mime,
      };
      // keep same property name as file stream
      // https://github.com/cojs/busboy/blob/master/index.js#L114
      meta.fieldname = meta.field;
      meta.transferEncoding = meta.encoding;
      meta.mimeType = meta.mime;

      ctx.coreLogger.debug('[egg-multipart:storeMultipart] handle stream part: %j', meta);
      // empty part, ignore it
      if (!part.filename) {
        await sendToWormhole(part);
        continue;
      }

      if (!storedir) {
        // ${tmpdir}/YYYY/MM/DD/HH
        storedir = path.join(ctx.app.config.multipart.tmpdir, moment().format('YYYY/MM/DD/HH'));
        const exists = await fs.exists(storedir);
        if (!exists) {
          await mkdirp(storedir);
        }
      }
      const filepath = path.join(storedir, uuid.v4() + path.extname(meta.filename));
      const target = fs.createWriteStream(filepath);
      await pump(part, target);
      // https://github.com/mscdex/busboy/blob/master/lib/types/multipart.js#L221
      meta.filepath = filepath;
      requestFiles.push(meta);

      // https://github.com/mscdex/busboy/blob/master/lib/types/multipart.js#L221
      if (part.truncated) {
        await ctx.cleanupRequestFiles(requestFiles);
        limit('Request_fileSize_limit', 'Reach fileSize limit');
      }
    } while (part != null);

    ctx.request.body = requestBody;
    ctx.request.files = requestFiles;
  },
```
![image](https://user-images.githubusercontent.com/88260489/144530462-555a7bf9-a939-4abf-a894-6f0a101b6a47.png)
导致该问题的原因是egg-multipart的saveRequestFiles()没有生成对应的part文件。