Document appendBlob for JS multipart form data: KTOR-1915

[e5l]

Context

PR: ktorio/ktor#5390
Issue: https://youtrack.jetbrains.com/issue/KTOR-1915

What changed

New JS-specific FormBuilder.appendBlob() extension functions were added to enable browser file uploads via the Ktor client multipart API. Previously, there was no way to create PartData from org.w3c.files.File or org.w3c.files.Blob in Kotlin/JS, making the documented multipart API unusable from browser environments.

What should be documented

The multipart form data request documentation should include a JS-specific section or note explaining how to upload files from a browser using appendBlob. The relevant module is ktor-client-core, specifically the io.ktor.client.request.forms package.

Suggested code snippet

// Upload a file from a browser <input type="file"> element
val file: File = // obtained from HTMLInputElement.files
client.submitFormWithBinaryData(url, formData {
    appendBlob("document", file, file.name, ContentType.Application.Pdf)
})

// Upload a raw Blob
val blob: Blob = // any Blob instance
client.submitFormWithBinaryData(url, formData {
    appendBlob("data", blob)
})

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