display only first characters - no diesel

Hi guys,

I'm looking for a content to show filename in my field but only the first 10 characters. I wish i could use Diesel here but Trueview doesnt support diesel.
Any solution to edit this: %<\AcVar Filename \f "%tc2%fn6">% without diesel to show only first 10 characters from filename?

another solution a .dwg viewer that Does support diesel?

thanks in advance.