linux mail < file.log has Content-Type: application/octet-stream (a noname attachment in Gmail)

The man page is a good place to start! Keep reading until you get to the MIME TYPES section, and pay close attention the following:

Otherwise, or if the filename has no extension, the content types text/plain or application/octet-stream are used, the first for text or international text files, the second for any file that contains formatting char‐ acters other than newlines and horizontal tabulators.

So, if your message contains "formatting characters" (which in general means control characters) other than newlines and tabs, it will automatically be classified as application/octet-stream. I bet that if you look closely at the data you'll find some control characters floating around.

You can work around this by...

• Including the log file as an attachment (using -a) instead of the main message body, and set up your ~/.mime.types file to identify *.log files as text/plain.
• Filter out control characters using something like tr.

• Use another MUA such as mutt to send the mail. In fact, you could just craft a message yourself and send it directly to sendmail:

  (  echo To: person@example.com  echo From: you@example.com  echo Subject: a logfile  echo  cat logfile.log) | sendmail -t

I got the similar problem recently and finally end up with a solution that is shorter:

cat -v log/logfile.log | mail -s "here is a log file" "person@example.com"

More details of the discussion of cat with mailx.

I had some trouble to get my automatic email scripts to run after changing to Ubuntu Precise 12.04. I don't know, when Ubuntu (or Debian) exchanged bsd-mailx against heirloom-mailx, but the two "mail"-commands behave very differently. (E.g. heirloom uses -a for attachments, while it's used for additional headers in bsd.)In my case heirloom-mailx wasn't able to reliably determine the Mime type and kept sending text as attachments. Blame me for not weeding out control characters or whatever, but I don't see much point in changing scripts that did their job perfectly before the upgrade. So if you prefer setting the Mimetype yourself, bsd-mailx is a better solution.

sudo apt-get install bsd-mailx sudo apt-get remove heirloom-mailx

Solved it for me.