Expires Headers Filename Vs CGI Params.

I am setting up Expires headers for my site. The site allows users to upload photos of themselves. The photos are stored as <user id number>.jpeg. I want these to be cached, but updated when the user uploads a new photo. From what I understand, new browsers use ETags to make sure this happens (since if they upload a new file, the inode and/or mtime will change). However for older browsers, I plan to add ?file-mtime to the end of the file name. This should allow caching but it will change when the user uploads a new image. Will the cause any technical problems with caches that would not be a problem if I changed the filename to be user_id.mtime.jpeg when the user uploads the file? The trade-off is that if I just append ?file-mtime to the end of the filename, I don't have to track these images in a database or use file globs (cpu-expensive for large directories) to find the filename to output.