Probably needs a more thorough redesign, but this'll do as a "hotfix".
<table border='0'><tr>
<td align='right'>{$sourcefilename}:</td><td align='left'>
- <input tabindex='1' type='file' name='wpUploadFile' id='wpUploadFile' onchange='fillDestFilename()' size='40' />
+ <input tabindex='1' type='file' name='wpUploadFile' id='wpUploadFile' " . ($this->mDestFile?"":"onchange='fillDestFilename()' ") . "size='40' />
</td></tr><tr>
<td align='right'>{$destfilename}:</td><td align='left'>