multipart/form-data should not contain content-type header

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Describe the bug

If post 'multipart/form-data' form by fetch, it should not contain content-type header, the boundary will be missing.

Related issues

https://stackoverflow.com/questions/39280438/fetch-missing-boundary-in-multipart-form-data-post

Which package/s are you using?

felte (Svelte)

Environment

• OS: windows
• Browser: edge
• Version: [[email protected]]

To reproduce

No response

Small reproduction example

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Screenshots

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Additional context

No response