Bearing in mind that I have not seen the code, if the code is uncompiled and is editable, there is probably a line of code that will represent the file extentions. This will be most likely be in a string.
Have a look in the code for ".jpg" and if you notice other extentions in the same string, just add the extentions you want.
There is also the posibility that the extentions are stored in the web.config or even an
xml file if it is a compiled control.
If you have some code, post it here so I / we can all have a look.
HTH