[#71200960] ia url generation should work with any filename

@baileyspace @kookster This should fix the generation of cdn urls for files stored in IA. 

When I removed the stubs on `AudioFile` in the spec, this had uncovered some issues with how I had written the string concatenation in `AudioFile#ia_url`. 

Now, instead of replacing only `.wav` extensions, I am replacing all extensions for all files stored in ia with `.mp3` taking the assumption that if a file is stored in ia, it's always going to be an '.mp3' after transcoding is completed, as per conversation with @baileyspace earlier today.

For files stored in s3, or for files originating elsewhere, this method is never called, as per `AudioFileUploader#url,` which only invokes `AudioFile#ia_url` when the provider is `InternetArchive`.