Encoding corrupts data

[andrew804]

Describe the bug

When using FormDataEncoder to encode data objects, specifically JPEG image data, the data gets corrupted.

To Reproduce

This can easily be reproduced by feeding JPEG image data into FormDataEncoder's encode function, then feeding it's output into FormDataDecoder's decode function and comparing the input data with the output data.

let folder = #filePath.split(separator: "/").dropLast().joined(separator: "/")
let path = "/" + folder + "/Utilities/image.jpeg"
let originalData = try Data(contentsOf: URL(fileURLWithPath: path))

struct ObjectToEncode: Codable {
    let data: Data
}

let object = ObjectToEncode(data: originalData)
let boundary = UUID().uuidString

let encoded = try FormDataEncoder().encode(object, boundary: boundary)
let decoded = try FormDataDecoder().decode(ObjectToEncode.self, from: encoded, boundary: boundary)

XCTAssertEqual(originalData, decoded.data)

The comparison fails indicating that the original data is not equal to the decoded data.

When inspecting the bytes it shows a ~90% increase in size. @Joannis suggested one can expect such increase with base64 encoding while @vzsg was more specific pointing out that base64's "expanded size" is 4:3 not 3:2.

@vzsg also expressed concern that the encode method returns a String instead of Data as binary files may not be valid UTF-8.

Expected behavior

The decoded data is equal to the original data.

Environment

multipart-kit version: 4.6.0
swift-tools-version: 5.7
macOS version: 14.2.1