File upload script WP2

Hy, I’m having a hard time finding the script, that handles file uploads. I’m looking for the part where file is written to new directory on server – can anyone help me finding that particular part?

I need this because my server is having problems with language-specific symbols (like ě??á, etc.) and file names containing them can’t be viewed after upload, because server redirects me to error-404 page – so i need to strip those symbols after upload.

Thank you in advance.