As far as I know, there is no easy way to do this.
I don't know if you can change types files when attached it .
For custom code option, I would do the following: create a field in SRD:Service Request Definition form where store file types for each SRD, and create custom workflow in SRM that validate if files extensions attached are allowed for this SRD.
Hope this helps.
I'm not aware of any way to limit what can be shown in the dialog, but in 7.6.04 SP5, and 8.1 (forget the patch), I know that you can specify black/white list extensions on what can or cannot be attached once selected.
Thanks for the tips, but they apply only after having selected a file extensions.
I'm really looking for a filter to apply on forehand, to limit the files shown when browsing