Prevent ImageField from overwriting existing files

[lacatoire]

The default upload_validate callable in FileUploadType checks file_exists() to avoid collisions, but it only received the file basename, so the check was always false and existing files were silently overwritten.

StringToFileTransformer now passes the full upload_dir + filename path to upload_validate and strips the prefix from the returned value, restoring the intended collision detection for both local and Flysystem storage.

Fixes #7525