Il tuo caso dovrebbe essere questo, spero che con l'inglese te la cavi meglio di me.
"Do this automatically..." checkbox disabled
Some mail programs send attachments with a nonspecific content type: application/octet-stream or application/x-msdownload. These content types can apply to various different types of file. Therefore the box "Do this automatically for files like this from now on" is disabled.
To work around this, ensure that Thunderbird's attachment-handling action for the file name extension is not the system default application. Instead, change it to specify the actual application.
If that doesn't help, send yourself an e-mail to which you manually attached a file with this file name extension. (Do not forward an email whose attachment caused the box to be disabled.) Then open the attachment and choose how you want Thunderbird to open it. Check the box "Do this automatically for files like this from now on".
Note: Another workaround is to set your operating system to handle these nonspecific content types using some application. This is not usually a good idea, because these nonspecific content types can apply to various different types of file.
Another way to get the checkbox enabled, if you know the correct content type, is doing following:
Save mail to disk
Open saved mail in text editor
Change "application/octet-stream" to the specific content type associated with the attachment's file extension.
Save changes and close text editor
Open saved mail in Thunderbird
"Do this automatically..." should now be enabled
http://kb.mozillazine.org/Actions_for_attachment_file_types#.22Attachments_Folder.22_settings
Il file mimetypes.rdf viene indicato come quello che memorizza le impostazioni ma mi sa che sia cambiato. Ho provato ad aggiungere un tipo di file e non lo ha modificato e cancellandolo le impostazioni che già aveva thunderbird sono rimaste.
Quindi direi dimenticati del file.
X Michro, la ricerca su google mi ha mostrato per primo l'altro articolo, Sorry.