Route every file through your own handler with UploadUrl
Point UploadUrl at an ASP page of your own and it receives each file as it arrives — validate it, rename it, store it anywhere, or reject it with a message that shows up in the queue. Allowed file types: jpg, gif, png, zip, up to 10 MB each.
UploadUrl
Custom handler
Live demo
The handler demo2_upload.asp saves each file as savefiles/myprefix_<name> and would reject a file named some.bmp with a custom error.
Read the guide
Chunked and resumable upload
How a custom handler receives large files without exhausting server memory.