Unable to upload pngs and jpeg with post-req

🐛 bug report

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Description of the problem

I am unable to upload png/jpeg, in a POST-request, and I'm getting an error of "Something went wrong".

How has this issue affected you? What are you trying to accomplish?

I am trying to dynamically create sandboxes, with content which is dependent on the file and it's own dependencies. For now I am unable to upload these related images.

To Reproduce

https://github.com/liroliro/csb-problem

Link to sandbox: link (optional)

I'm unable to create a sandbox, since I can't run scripts within it.

Your Environment

| Software | Name/Version |
| Catalina | 10.15.7 |
| Сodesandbox | |
| Browser | Chrome |
| Operating System | Mac |

Start with the reproduction repository at https://github.com/liroliro/csb-problem and trace the POST upload path for PNG and JPEG files. Reproduce the "Something went wrong" error, then identify the relevant upload entry point; done means both image types upload successfully through the POST request.