[imp] ACAD files (DWG) attached as plain text - Problem with mime.types or magic

[Dop's Mort's]

When I attach files of the autocad (.dwg) they are transformed in plain text by the IMP.  
The IMP don't recognize this file type.  

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Solution: The browser and/or the mime magic detection on your server is responsible for setting the correct mime type.  

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Questions :
What file should I update ? 
/etc/mime.types ?
/usr/share/magic.mime ?
/usr/share/file/magic.mime ?
/etc/httpd/conf/magic ?

What lines should I add to this files ?

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I know this isn't a bug from Horde, but I already configured some files and still got error. I didn't find anywhere what the right lines and files.
Running on Linux (apache)
Tks.