change buffer type to string in File interface

[JunNishimura]

What I did

This fixes grafana/k6#4655

Regarding the above issue, I took the approach of changing the type of the buffer property of the File interface to string.

I thought that aligning the type to Base64-encoded strings would have the smallest impact because the existing implementation expects Base64-encoded strings.

• Buffer field of File Type in k6 expects to be string
  • https://github.com/grafana/k6/blob/3fdfa3baa7e561f05d075cc5eb1e9633a609a109/internal/js/modules/k6/browser/common/element_handle_options.go#L95
• Unit tests expect to have Base64-encoded strings for buffer property
  • https://github.com/grafana/k6/blob/master/internal/js/modules/k6/browser/tests/setinputfiles_test.go
• In injected_scripts that actually create files within the browser, the code is decoded using atob(file.buffer).
  • https://github.com/grafana/k6/blob/master/internal/js/modules/k6/browser/common/js/injected_script.js
• The documentation also indicates that base64 encoded data is required.
  • https://grafana.com/docs/k6/latest/javascript-api/k6-browser/elementhandle/setinputfiles/
  • https://grafana.com/docs/k6/latest/javascript-api/k6-browser/frame/setinputfiles/
  • https://grafana.com/docs/k6/latest/javascript-api/k6-browser/page/setinputfiles/

The description regarding mimeType mentioned in the issue has already been resolved, so I didn't do anything about the mimeType property.

The issue where the type of the buffer property is listed as ArrayBuffer in the documentation has been fixed in the following pull request. I plan to merge the PR below after merging this PR, or at the same time.

grafana/k6-docs#2205

---

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[joanlopez]

Can you please review the conflicts @JunNishimura? Thanks!

[JunNishimura]

@joanlopez

Hi, I resolved the conflicts. I would appreciate it if you check again. Thanks!