It seems the ultimate culprit for the problem is Finder and the way it decides how to handle a file when clicked and what to show to the end user as an icon. For the icon it seems simple, Finder will look at the extension and select the icon based on this and will always use this.

Now for the scary bit, to decide what to do with the file when the file is clicked on. Finder will take a look at the file permissions and then go from there, if it has any of the executable bits set it will get the Terminal app to execute the file. If no executable bits are set then it will use the application appropriate to the file extension to open the file.

Ok, so not hugely scary just yet, but it gets worse. If sent via email it can get nasty, why? Well to have this do nasty stuff the email is sent with a Content-type of multipart/appledouble for the MIME attachment, here in lies the problem. The Mac Mail client will parse this content type and there are some parameters in there that can be used to set the permissions on the file when handled by the Mac Mail client.

--Apple-Mail-4--991165126
Content-Transfer-Encoding: 7bit
Content-Id: <
 This e-mail address is being protected from spambots. You need JavaScript enabled to view it
 >
Content-Type: image/jpeg;
	x-mac-type=0;
	x-unix-mode=0755;
	x-mac-creator=0;
	name="Heise.jpg"
Content-Disposition: inline;
	filename=Heise.jpg

/bin/ls -al

Luckily Mozilla Thunderbird does not honour the apple MIME extensions and therefore Thunderbird users are safe from this little bug.

Meanwhile, if your continuing to use Mac Mail, be careful, never double click an attachment, save it first, then go to the location of the file and check out which application will really open the file using the kind column. Also if you can do this, strip the x-unix-mode MIME option from incoming mails.

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