Thanks for a bug report. Even if I fix the content type of the attachment by adding image/tif and image/tiff into [1], the webkit isn't able to show the tiff file, which is a pita. I can right-click the "missing image" icon and choose "copy image", then paste it into for example GIMP, and it is shown properly, thus it's something with the webkit not supporting tiff views for some reason (at least on my side).

Tom, do you know whether webkitgtk+ is able to show TIFF images inline, please?

[1] https://git.gnome.org/browse/evolution/tree/em-format/e-mail-parser-image.c#n40

(In reply to Milan Crha from comment #1)
> Tom, do you know whether webkitgtk+ is able to show TIFF images inline,
> please?

It actually supports it (Firefox and Chromium does not). You can try to load [2] in GtkLauncher to try it for yourself. But it needs to be specially embedded in the page as you can see in [2]. We should probably go with example 6 or 7.

[2] - http://www.alternatiff.com/howtoembed.html

(In reply to Tomas Popela from comment #2)
> (In reply to Milan Crha from comment #1)
> > Tom, do you know whether webkitgtk+ is able to show TIFF images inline,
> > please?
> 
> It actually supports it (Firefox and Chromium does not). You can try to load
> [2] in GtkLauncher to try it for yourself. But it needs to be specially
> embedded in the page as you can see in [2]. We should probably go with
> example 6 or 7.
> 
> [2] - http://www.alternatiff.com/howtoembed.html

And actually once we implement this it will stop to work in WebKit2 port as we want to disable the plugins there (you can try it in MiniBrowser by going to preferences, disable plugins there and reloading the page).

Created attachment 359369 [details] [review]
evo patch

for evolution;

As Tomas said, with WebKit2 the TIFF images cannot be shown inline, thus let's disable them for inline view and offer only Save and Open with... items.

Created commit f04d69a in evo master (3.27.1+)
Created commit acff211 in evo gnome-3-26 (3.26.1+)