Attachments behaviour on ticket forward

hi everyone,
(on RT version 3.8.8, sending via smtp command)

when forwarding a ticket with several attachments, i have the following
behaviour :
a zz.xls file is attached correctly as a xls file
a xxx.TXT file is displayed in the body
a yyy.log (containing plain-text) file is attached as a yyy.dat file

theses are 3 differents things depending on the file type/extension.
what i want is to always attach the files with original filename/extension

where are the config option to setup attachments behaviour ?
I haven’t found any revelant option in RT_SiteConfig / RT_Config files …
may it be in the linux smtp configuration ?

Thank you,
Rapha�l
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