What this PR does / 这个 PR 做了什么

On React Native, uploadFile builds the multipart file object with a hardcoded name: 'file', so the server always receives the file name file and cannot read the original name or extension (e.g. to distinguish image types), as reported in #19505.

RN 端 uploadFile 组装 multipart 文件对象时把 name 写死为 'file'，后端拿不到真实文件名/后缀（#19505）。

The H5 implementation already sends a meaningful file name (form.append(name, fileObj, fileName || \file-${Date.now()}`)), and the public Taro.uploadFile.Optiontype already exposesfileName`. RN is the outlier.

How / 修改方式

packages/taro-rn/src/lib/file.ts — createFormData now resolves the file name as:

1. the explicit fileName option, then
2. the filePath basename (e.g. xxxx.jpg), then
3. the historical 'file' fallback when neither exists.

packages/taro/types/api/network/upload.d.ts — fileName 的 @supported 标记从 h5 更新为 h5, rn（该选项类型本来就存在，仅文档标记未覆盖 RN）。

Verification / 验证

• Red: with the fix reverted, the two new assertions fail (name is always 'file'); the fallback case passes on both old and new code, pinning backward compatibility.
• Green: packages/taro-rn full Jest suite — 14 suites / 36 passed / 1 skipped, including the new __tests__/uploadFile.test.ts (basename derivation, explicit fileName override, 'file' fallback).
• tsc --noEmit on packages/taro-rn: zero errors in changed files (the single remaining error is the pre-existing @tarojs/runtime-rn workspace build-order issue in untouched src/api/index.ts, identical before and after).
• ESLint on changed files: 0 errors; git diff --check clean.

Note: the package's own testMatch glob does not match on Windows (mixed separators), so tests were run with npx jest --testMatch '**/__tests__/**/*.test.ts' — same suites, no repo config changes.

Fixes #19505

Summary by CodeRabbit

• 新功能
  • React Native 平台的文件上传现支持自定义 fileName。
  • 未指定文件名时，系统会优先使用文件路径中的名称；无法解析时回退为默认名称。
  • 上传文件类型统一标记为 application/octet-stream。
• 测试
  • 新增文件上传相关测试，覆盖自定义文件名、路径名称提取及默认名称回退场景。