Happens to me too on LXDE Ubuntu 14.04, same behavior, same bug.
I have tried with Gnome Shell 3.12.2 and all works fine.
I've open some thread about this:
One on LXDE Forum without any answer:
Another one on Linuxquestions:
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Simple Scan does not attach files in Thunderbird
Status in One Hundred Papercuts:
Status in xdg-utils package in Ubuntu:
Steps to reproduce:
1- Set Thunderbird as default email client
2- Open Simple Scan and scan a document
3- Choose Document -> E-mail
3- Document does not get attached in Thunderbird
This happens in Trusty and later.
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