xref bug 276991. If an message contains a link to an attached part, clicking the link is handled according to the part's filename rather than its content-type. This is not necessarily a problem, because the handling comes back thru Mozilla rather than being launched. It appears the behavior is: Look up the MIME type associated to the extension in the registry Look up the MIME type in Mozilla's Helper Applications However, in certain messages, it is possible to view the attachment in the attachment panel (in the plain-text portion); and double-clicking on that will open the file according to the Content-Type associated. Test case follows.

This message contains a text/plain and text/html parts (as alternatives) and an 'related' .WAV file which is mis-specified as "application/pdf". To reproduce: 1) Integrated message into email (Moz 1.7+). 2) Configure Helper Applications with: application/pdf: Open With Default App audio/wav: <no entry> 3) Configure system registry such that HKCR\.wav has a string value Content-Type = audio/wav 4) View the message As Original (or Simple) HTML 5) Click on the link. Get a "Mozilla does not know how to handle audio/wav" message; instruct it to "Open With Default App", always henceforth 6) View the message As Plain Text. 7) Double-click the attachment icon Actual results: 5) Mozilla claims that the linked data is type "audio/wav" (determined via file extension and system registry), plays sound eventually 7) Mozilla attempts to open WAV file in Acrobat, gives error Expected results: 5) *and* 7) Mozilla attempts to open WAV file in Acrobat, gives error

I guess, looking at other bugs related to Windows extension handling, this should be a major bug too. CC'ing some people familiar with the problem domain.

It sounds like the mailnews channel in question just doesn't report the content-type specified in the part... I'm pretty sure we have existing bugs on this, languishing.

sorry for the spam. making bugzilla reflect reality as I'm not working on these bugs. filter on FOOBARCHEESE to remove these in bulk.

Still repro. Thunderbird 52.0.1 (32-bit) Windows 7 64-bit