(Swift 3,4,5...) How to Tell if a File is .p7m or .p7s

This example explains how to detect if a file is a .p7m or .p7s. It provides as solution to the following problem:

Sometimes people submit P7S/P7M with wrong extensions. Can I check if file is P7S or P7M with Chilkat?

Also see What is a P7M or P7S File?

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func chilkatTest() {
    // A .p7m or .p7s file is binary and contains PKCS7.
    // PKCS7 is ASN.1, and always begins with a SEQUENCE tag (a byte equal to 0x30)
    // followed by an encoded length.
    // If the file is larger than 128 bytes (and it SHOULD be), the encoded length will begin with either 0x80, 0x81, 0x82, or 0x83.
    // It can also begin with 0x84, but only for very large files (where the length is too large for 3 bytes).
    // See How ASN.1 Encodes Lengths

    // Therefore, if we have a file that might be .p7m or .p7s, we can check by
    // examining the 1st 2 bytes of the file.

    let bd = CkoBinData()!

    var success: Bool = bd.loadFile("qa_data/p7m/a.p7m")
    if success == false {
        print("Failed to load the file.")
        return
    }

    let sb = CkoStringBuilder()!
    sb.append(bd.getEncodedChunk(0, numBytes: 2, encoding: "hex"))

    var caseSensitive: Bool = true
    if sb.contentsEqual("3080", caseSensitive: caseSensitive) || sb.contentsEqual("3081", caseSensitive: caseSensitive) || sb.contentsEqual("3082", caseSensitive: caseSensitive) || sb.contentsEqual("3083", caseSensitive: caseSensitive) {
        print("This is a .p7m or .p7s file.")
    }
    else {
        print("This is not a .p7m or .p7s file.")
    }


}