Does someone know if it is possible to have proftpd to behave like this:
If a file is replaced but upload failed, then keep the successfull old file.
To keep file integrity. It's because i'm using it with an app.
For this, I would recommend using the following in your proftpd.conf:
HiddenStores on
The HiddenStores behavior sounds like what you want: the file being uploaded by the client is stored by proftpd in a temporary "hidden" file. When the upload completes successfully, that temporary file is atomically renamed into place. If the upload fails, then the temporary file is deleted.
Hope this helps!