Missing MIME type in attachments
Thunderbird relies solely on the MIME type for determining what application should be used to open an attachment. Normally this isn't much of a problem, however if the the mail agent used by the sender failed to set the MIME type correctly (something that yahoo.com web mail sometimes does) Thunderbird will not be able to determine what to do with the attachment. The 'Open Attachment By Extension' extension (see below) solves this.
Open Attachment By Extension
The Open Attachment By Extension extension
enables Thunderbird to open attachments based solely on file name extension. Thunderbird relies on the MIME type to determine what application to use, however when the MIME type is missing Thunderbird has no way to determine what application to run, thus this extension saves the day! In my opinion this should be addressed in the actual Thunderbird program by the developers, yet that doesn't seem to be something they are currently interested in 'fixing'.
Unfortunately there is no graphical configuration window for this extension, however all that is needed is to edit the file user.js (located in ~/.thunderbird/X/, where X is a randomly created directory name, create the file if you don't find it) adding entries similar to the following:
user_pref("openattachment.extension.txt", "/usr/bin/gedit"); /* text files -> gedit */
user_pref("openattachment.extension.doc", "/usr/bin/ooffice"); /* ms word -> ooffice */
user_pref("openattachment.extension.xls", "/usr/bin/ooffice"); /* ms excel -> ooffice */
user_pref("openattachment.extension.pdf", "/usr/bin/acroread"); /* adobe acrobat -> adobe acrorread */
The MD5 of the 0.1.2 version (OpenAttachmentByExtension-0.1.2.xpi) is 5309948c9470352debbae11e1cd36e2e.