An event OnBeforeFileUpload has been suggested. Right now all there is is the OnFileManagerUpload event, but that is evoked after the file is uploaded and moved into place.
From the core/model/modx/sources/modfilemediasource.class.php file
if (!move_uploaded_file($file['tmp_name'],$newPath)) {
$this->addError('path',$this->xpdo->lexicon('file_err_upload'));
continue;
}
}
/* invoke event */
$this->xpdo->invokeEvent('OnFileManagerUpload',array(
'files' => &$objects,
'directory' => $container,
'source' => &$this,
));
An OnBefore... event could be added just before the move_uploaded_file() function call, so then a plugin could do things like check for an existing file of the same name and modify the name to save the file by.