The Convert image by domain add-on will convert hot linked images in messages to attachments. Hot linked images are those images that have been inserted using the Image icon in the editor toolbar.
(Example of IMG tag)
View attachment 9490
(Example of ATTACH tag)
View attachment 9491
(Example of Maximum attachment image dimensions selection)
View attachment 9492
(Example of Options page)
View attachment 9493
(Example of Convert image...