MimeType.swift

import Foundation
import MobileCoreServices

extension Data {
  internal var imageMime: String? {
    let start = (self as NSData).bytes.bindMemory(to: UInt8.self, capacity: self.count)

    guard let byte: UInt8 = UnsafeBufferPointer(start: start, count: 1).first else { return nil }

    switch byte {
    case 0xFF:
      return mimeType(uti: kUTTypeJPEG)
    case 0x89:
      return mimeType(uti: kUTTypePNG)
    case 0x47:
      return mimeType(uti: kUTTypeGIF)
    default:
      return nil
    }
  }
}

extension URL {
  internal var imageMime: String? {
    return mimeType(extension: self.pathExtension, where: kUTTypeImage)
  }
}

private func mimeType(extension: String, where: CFString? = nil) -> String? {
  let uti = UTTypeCreatePreferredIdentifierForTag(
    kUTTagClassFilenameExtension,
    `extension` as CFString,
    `where`
  )?.takeRetainedValue()
  return uti.flatMap(mimeType(uti:))
}

private func mimeType(uti: CFString) -> String? {
  return UTTypeCopyPreferredTagWithClass(uti, kUTTagClassMIMEType)?.takeRetainedValue() as String?
}