Hi oldturkey03-
Basically, the difference is that you are saying that the TP won't open the file because the relevant application is not installed. That may be the case sometimes. But the TP also has a more fundamental fault in that often it will also not open files for which it does have the relevant file installed. For instance, it oftens happens when trying to access pdfs when Adobe Acrobat is installed and .doc files when Quick Office or a.n.other is installed.
As you correctly say a file extension (.doc, .pdf etc) is associated with a file type, so that your browser knows how to work with it. But with the TP is will often download the file with an .aspx extension instead of interpreting the file correctly. The .aspx (Active Server Page Extended) file extension stores script and source codes that allow web browsers to translate and open web pages. Virtually all web browsers can 'decode' the script and then use the correct application to open the file. The TP browser fails to do so. Instead it just downloads the .aspx and just goes 'huh, what am I supposed to do now?' and gives you the MIME error.
Currently there are only two real solutions:
1) Install Preware on your TP and then install 'Internalz' - a file management application. You then download the document you want to open. Navigate to the document using Internalz and alter the file extension from .aspx to whatever it should be (.pdf, .doc etc). Then choose to open it and - hey, it works.
2) Install Android and your TP and forget about WebOS.
Finally Devwithoutpower on WebOS Nation has created a solution apparently - see http://forums.webosnation.com/native-apps/308020-solved-web-secure-downloads.html
However, I have tried it and it won't work for me. There's some confusion as to whether it works since the 3.0.5 WebOS update, which I run. So personally - I just use Android now.