Have a query regarding how DNN handles links to files internally stored on it's file system.
The system being used is still running DNN4.9 whilst we are redeveloping for DNN6.

Since all the links to internal directories/files must presumably go through the main default.aspx handling I am wondering where to look for this info.

Specifically if I have an attachment that is for example a jpg in my file system say for example Portals\0\Attachments\nicelogo.jpg and build a url link to it such as "http://<mydomain>/Portals/1/Attachments/nicelogo.jpg" it all works nicely and I can see the image in my browser.
To my knowledge this also works perfectly in regards to PDF extensions, DOC extensions etc etc.

However if I change the host settings to allow someone to attach an mp4 extension the url "http://<mydomain>/Portals/1/Attachments/shortvideo.mp4" causes a 404/file not found error. 

Now I understand that HOW a file displayed in a browser is dependent on the browser etc - however the 404 error also means that Save Target etc are not working.

Having piled through aspects of the DNN source code I notice that in most DNN4 or previous versions none of the core libraries have mp4 in their content-type conversion/lookup functions. 

Is this the cause of the 404?  Is there a way to make an mp4 not return a 404 error? I don't care about how it presents on a browser, just want to make it accessible and downloadable once uploaded.

Thanks