This may be a feature request or may be a buglet introduced in 3.4.x.
I tend to use file names like this --> Dash 3.7 Plate work3.dxf
When I open it, usually from the Recent Files option, loads fine.
I make mods, decide I want to save a new revision, lets say Dash 3.7 Plate work4.dxf
With QCAD 3.4.x, If I click the SaveAs dialog, the field pre-fills with Dash 3.dxf, so I have a lot of typing to do to get my desired new file name.
Apparently (presumably) the embedded "." in the file name causes this.
I did a lot of work a year ago, running 3.2.x and then 3.3.x and I seem to recall that SaveAs would start with to full current file name, Dash 3.7 Plate work3.dxf, making it much easier (and less subject to typos) to rev the name.
Unfortunately, I did not keep my old 3.2 version around for test purposes.
If my recollection is incorrect, consider this a feature request; it is very convenient.