Tested each sample.

gem/ex12.lua

@@ -0,0 +1,34 @@
+local smtp = require"socket.smtp"
+local mime = require"mime"
+local ltn12 = require"ltn12"
+
+CRLF = "\013\010"
+
+local message = smtp.message{
+  headers = {
+    from = "Sicrano <sicrano@example.com>",
+    to = "Fulano <fulano@example.com>",
+    subject = "A message with an attachment"},
+  body = {
+    preamble = "Hope you can see the attachment" .. CRLF,
+    [1] = {
+      body = "Here is our logo" .. CRLF},
+    [2] = {
+      headers = {
+        ["content-type"] = 'image/png; name="luasocket.png"',
+        ["content-disposition"] =
+          'attachment; filename="luasocket.png"',
+        ["content-description"] = 'LuaSocket logo',
+        ["content-transfer-encoding"] = "BASE64"},
+      body = ltn12.source.chain(
+        ltn12.source.file(io.open("luasocket.png", "rb")),
+        ltn12.filter.chain(
+          mime.encode("base64"),
+          mime.wrap()))}}}
+
+assert(smtp.send{
+  rcpt = "<diego@cs.princeton.edu>",
+  from = "<diego@cs.princeton.edu>",
+  server = "localhost",
+  port = 2525,
+  source = message})