Copy and paste is the anticipated method.

It seems a bit odd these days to be considering taking text from a plain text file. And if your donor file has any formatting markup, there are likely to be inconsistencies in how it interacts with Thunderbird formatting. I'm curious as to the workflow or use case here; what's the reason for doing this? Are you using a small number of text files for standard replies? If so, you could use an add-on to paste regularly-used text snippets into your messages.

Note that if your recipients use Thunderbird, they can, if they wish, see the content of plain text attachments (and certain graphics files) displayed inline below your message text, so they don't need to do anything special to view the attachment. But is a double-click to open a text file in the default editor or viewer really so onerous?

Having said that, I was today amazed and irritated to receive by RSS feed a notification of a notice, which was presented as a link to a pdf document. The link took me to a webserver which offered up the pdf document. The linked document, in turn, provided an email address that should be used to make a specific kind of request. The email address was non-clickable and it seems that the pdf document was a graphic, possibly scanned from a hard copy on paper; it had that grey unsaturated printing typical of a scan. The original printed document is likely pinned up on a notice board. My immediate reaction was to wonder why I had to download the pdf document and why they hadn't just posted the original notice's text into the feed message. It would have been less effort for me to read, and I'd also have been able to copy the quoted email address straight to my address book. I put it down to a mix of ignorance and incompetence; they had set about creating a poster and later decided to disseminate it more widely via RSS, but didn't bother to create a new message based on the text of the original document.

It's a bit like those folk who want to send you a picture or two and they offer you a Word document with the images embedded in it, because they don't know how to work directly with graphic files.

Thank you, I appreciate the response.

I often take notes or otherwise writeup things in text files. It's a whole lot faster to fire up vi than it is libreoffice; and I already have libreoffice running 24/7 (unless it crashes). Once written, it sits... until someone says something to me, and I remember I have notes on that subject - and so I bring up thunderbird (also running 24/7) and create an email - at which point I want to insert that text file, inline. For that matter I'd like to be able to insert other types of text documents inline - pdf, doc, docx, odt.

Note that copying text from libreoffice to thunderbird doesn't work for me, I don't know why or what else to say about it. I copy the text in LO, and when I go to paste into thunderbird nothing happens. So even if I liked that solution, it doesn't work for me.

I've seen Thunderbird refuse to accept pastes; I usually work around it by pasting to an intermediary (Notepad++ here, but I've used Gedit, Pluma, Bluefish and vim as well) and then copy-and-pasting to Thunderbird. (Sorry, yes, this is even worse than where we started from!) I hadn't noticed this happening recently, but then again, I don't do this copy-and-paste thing very often. So I was thinking it might have improved with a recent update.

Please don't hold your breath waiting for the ability to paste all those other document types; we don't have support for .txt yet, so .doc, .odf etc must remain flights of fancy. :-(

Do you do this: Open text file Ctrl + A to highlight all text Ctrl + C to copy

In Thunderbird Write window Ctrl + V to paste.

Seems quite a quick method, only a few clicks and took me less than 5 seconds in a test.

However, just a thought which you might like to try...have you considered making notes in a Write compose window and saving as a Draft. Then the information is already in a 'saved' draft message. I use this because I can always use the right click and 'Save As' and save the draft as a text file for saving elsewhere on computer and still have info already for use in Thunderbird without needing to do all that copy pasting.