I’ve taken a look and it’s definitely a bug based on . in the filename. The composer is trying to just remove the extension so we end up with bridge-2-a5-flier.04|WIDTHxHEIGHT in the markdown, but it is just splitting the filename on the . and taking the first part of the file before the extension, which is obviously not correct.
From what I can see this bug has been present for two years, so thank you for being 
-eyed and picking it up! I will push a fix today.