I’ve spent a comfortable few days being usefully productive, email-wise, thanks to my recent switch from Apple’s Mail to Mozilla’s Thunderbird.

Then today, I needed to send an attachment for the first time since the switch. I was filing an article for publication on this very web site.

My editor responded with positive vibes about the words, but a puzzled question too: “Could you send me a clean attachment, though?”

Huh? I thought I had.

It turned out that Thunderbird was sending all attachments inline, embedded in the message body, rather than as proper attached files, the behavior I’d intended.

It also turned out, after a quick flick through the preferences, that this isn’t something you can toggle on and off. Off I went a-Googling.

You probably know where this is heading. The solution lies in the habit of Mozilla-based software to use something called User Profiles. Every user of Thunderbird is assigned a Profile, which is a series of files controlling how the application has been set up to behave.

There’s usually a prefs.js file, in which the application stores all your preferences.

And there’s an optional user.js file, which you can create in any plain text editor, where you can add your own preferences.

So, to force Thunderbird to send attachments the way I want it to, all I had to do was create a file called user.js in my user profile directory, and insert this line:

user_pref("mail.content_disposition_type", 1);

Hey presto! Now attachments are attachments once more.

I post this here purely for the benefit of future Thunderbird users who encounter the same issue and, after their own attempts a-Googling, fail to find the helpful Mozilla page that tells all.

But with any luck, a future version of Thunderbird might include a simple GUI switch to control this behavior, and users will be able to rest at ease without worrying themselves about obscure text files hidden in the darker corners of their hard drives.