regexp_replace question / help needed

I am running into a problem and need some pointers on regexp_replace - I can't seem to find an answer in any of the online resources.

I have a string (like 40,000 with different length and number of components) of them in a field named "externalurl". I need to replace the final "/" of the string with "&file=" while preserving the filename and extension following the "/".

The closest I can get is:

regexp_replace('http://test.com/test/testfile.php','/[^/]*$','&file=') 

however this looses the file name and returns:

What I am looking for is:

as a result.

Would anyone here point me in the right direction?

Thanks!

-Chris