User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0 Build ID: 20170517122419 Steps to reproduce: View attached message in plaintext mode on Linux using TB 52.1. Actual results: Thunderbird shows only the blue the image. Expected results: TB should display "This is the plain text part" and list the image filename in the attachment pane. It should not render any image at all. If you view in plaintext mode, you expect the display of text only. And if TB cannot display text for whatever reason, it must display a blank page. Under no circumstance may TB render images in plaintext mode. That is a sensitive security issue for people who trust "plaintext" will disable rendering of all images, which potentially include malware and the likes.

duplicate?

Indeed.