This would be nice so that users could use the system filechooser on graphical mode, and have tab completion on text mode, rather than writing a freeform string.
The 'file doesn't exist yet' usecase should be usable, too, so i guess GUI mode still needs a text field, but most filechoosers have those (haven't seen one without yet on linux).
A simple validation that the thing is indeed a validfile name would be nice too, at least for the CLI-mode.
ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: debconf 1.5.46ubuntu1
ProcVersionSignature: Ubuntu 3.5.0-21.32-generic 3.5.7.1
Uname: Linux 3.5.0-21-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.6.1-0ubuntu9
Architecture: amd64
Date: Tue Jan 1 19:34:08 2013
InstallationDate: Installed on 2011-12-25 (372 days ago)
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: debconf
UpgradeStatus: Upgraded to quantal on 2012-10-20 (73 days ago)
With the amount of work this would require at various levels of the stack, this is fairly unlikely to happen soon; but a specific use case where a file type would be compellingly useful would help.