[Denemo-devel] Export: Unnecessary warning and filename

[Nils Gey]

The current export (for example midi) still warns about "not .denemo
file" and afterwards changes the filename to .midi. 

In my understanding exported files are one-way saves. You export but
keep working in your original .denemo file.

Therefore the warning is not needed anymore and you should not work in
a midi file afterwards. (however this is done anyway...)

Nils