Apparently in Rails you need to manually set the MIME-Type of attachments, otherwise their filesize, filetype and other Metadata are not properly recognized by the E-Mail client and images will not be displayed within the html e-mail.
In this case using the hash :mime_type and the corresponding value 'image/png' in the rails controller solved this problem:

attachments.inline["img.png"] = {:data =>'img.png'), :mime_type => 'image/png'}


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