{assign var="aFile" value="somefile.xyz"|substr:0:-4|cat:".ext"}
<!-- Result expected: somefile.ext -->
<!-- Actual result:   somefile -->
{$aFile}

<br>

{assign var="aFile" value="somefile.xyz"|substr:0:4|cat:".ext"}
<!-- Result expected: some.ext -->
<!-- Actual result:   some.ext -->
{$aFile}

<br>

{assign var="aFile" value=("somefile.xyz"|substr:0:-4)|cat:".ext"}
<!-- Result expected: somefile.ext -->
<!-- Actual result:   somefile.ext -->
{$aFile}

<br>

{assign var="aFile" value="somefile.xyz"|substr:0:-4}
{assign var="aFile" value=$aFile|cat:".ext"}
<!-- Result expected: somefile.ext -->
<!-- Actual result:   somefile.ext -->
{$aFile}

<br>

{assign var="aFile" value="somefile.xyz"|substr:0:-(4)|cat:".ext"}
<!-- Result expected: somefile.ext -->
<!-- Actual result:   somefile -->
{$aFile}

<br>

{assign var="aFile" value="somefile.xyz"|substr:0:(-4)|cat:".ext"}
<!-- Result expected: somefile.ext -->
<!-- Actual result:   somefile.ext -->
{$aFile}


<br>

{assign var="aFile" value=-4|cat:".ext"}
<!-- Result expected: -4.ext -->
<!-- Actual result:   -4.ext -->
{$aFile}

<br>

{assign var="aFile" value=-4-4|cat:".ext"}
<!-- Result expected: -8.ext -->
<!-- Actual result:   -8.ext -->
{$aFile}

<br>