Okay, so assuming that [+navFilename+] is being set somewhere as a placeholder, could you adjust the Includer snippet to have an optional &ph option, like:
[[IncluderSnippet? &file=`assets/somefolder/views/nav/` &ph=`navFilename`]]
And then, check for that parameter in the Includer snippet using something like:
...
$ph = isset($ph) ? $modx->getPlaceholder($ph) : null;
if ($ph != null) $file = $file.$ph;
...
I’m not fully understanding if
navFilename is a TV or a placeholder, I’m going with placeholder seeing as [+navFilename+] works for you. A bit more info on how you are calling/processing the chunk would be useful if the above doesn’t work.