fix(attachments:sandbox): fix the empty attachment bug on macos

[sassman]

when having a sandboxed app, and using NSOpenPanel for letting users the selecting a file, the attachments did contain sizeof file times zero bytes.

Using the FileManager to access the file contents did solve the problem.

Also the base64 and cutting off after n characters is now simplified

[RockfordWei]

@sassman thanks for the PR. would you please provide some test scripts against it to prove it works, if possible?

[RockfordWei]

@sassman I can't build your code for these errors. Please let me know your thoughts. Thank you.

/Users/rockywei/Downloads/tmp/mactest/Perfect-SMTP/Sources/PerfectSMTP/PerfectSMTP.swift:265:2: error: source control conflict marker in source file
<<<<<<< HEAD
 ^
/Users/rockywei/Downloads/tmp/mactest/Perfect-SMTP/Sources/PerfectSMTP/PerfectSMTP.swift:251:19: error: use of unresolved identifier 'paramterRfc2231'
                        let filename = paramterRfc2231(something: file)
                                       ^~~~~~~~~~~~~~~
/Users/rockywei/Downloads/tmp/mactest/Perfect-SMTP/Sources/PerfectSMTP/PerfectSMTP.swift:252:15: error: use of unresolved identifier 'contentTypefilenameParameter'
                        let name = contentTypefilenameParameter(something: file)