fix Content-Type bug in NIP-96 uploadFile

[syusui-s]

Why

The NIP-96 uploadFile has a bug. This PR fixes it.

When using fetch API along with FormData, specifying Content-Type will cause error. It is because if we specify Content-Type: multipart/form-data, the browser will send it as is without considering "boundary".

Warning: When using FormData to submit POST requests using XMLHttpRequest or the Fetch API with the multipart/form-data content type (e.g. when uploading files and blobs to the server), do not explicitly set the Content-Type header on the request. Doing so will prevent the browser from being able to set the Content-Type header with the boundary expression it will use to delimit form fields in the request body.
https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest_API/Using_FormData_Objects#sending_files_using_a_formdata_object

The browser will detect Content-Type from body and append boundary=... appropriately. We don't need to specify Content-Type.

References

• https://stackoverflow.com/questions/39280438/fetch-missing-boundary-in-multipart-form-data-post
• https://zenn.dev/kariya_mitsuru/articles/25c9aeb27059e7
• https://nostr.com/note1xyk98avw48l3zm60fj8vesnuduujh3mxhs0px7l05dvlcpjjzdzswvas3x
• https://nostr.com/note1tz3v2d83ayk3uqa62nj64gefdpxu9vxr9jjd502fxelnmap9fqdq0tsghf