How can I insert text from a file into Thunderbird and make it appear as if it were typed text (as opposed to a hyperlink (which won't work unless I mail it to myself, since the file involve stays on my computer)? It is often too big to copy & paste. Also copy & paste often has glitches (like strange insertions of newlines and such). It is the awkward and messy way to do it that I am trying to get away from.

I remember some mail agent I used in the past that has this. But I don't find it in Thunderbird. Maybe it was Evolution that had that (but changing to Evolution is out of the question, now). Please treat suggestions of changing mail agent as off topic.

It's a shot in the dark, but I have had trouble pasting in Thunderbird and found that the option to use was "Paste without formatting." Just choosing "Paste" from the menu had strange (or no) results.

Frankly, I think Thunderbird would serve its users better if the two choices were "Paste" and "Paste with formatting," so that "Paste" would work in the conventional fashion.

I think maybe the problem is that the COPY step is working wrong when the text wraps, but happens to have a space at the end of the line. And if the document is larger than a terminal window, I'm screwed. But I did try your suggestion and an odd thing happened. First I did a regular paste with some text. Then I chose the "Paste Without Formatting". It pasted something else that I had done a copy on earlier today, not the text I had just copied shortly before trying this. Something is really wrong with that. I tried the other one "Paste As Quotation" and it also got older data.

So I think it still needs an "Insert File As Text" option.

When "Paste Without Formatting" doesn't do what I want in Thunderbird, I paste into a plain text editor (Xfce's Mousepad) and copy and paste from there.

I'll have to try the Gnome equivalent. Maybe it will work by having that plain text editor load the text file and do select all to copy it and paste that all into Thunderbird. Will try that next time I have one of these to do. I hope it can handle copy and paste largish (100k) text files.

Edit:

I tested on a smaller file I had handy and it comes out just fine. I suspect the "Paste Without Formatting" is to take the rich text attributes off like font glyph, font size, color, italics, etc. Normally those end up staying on if originally copied from something like a web browser or word processor.