When a sandboxed (i.e. signed with entitlements) application is run on Mac OS X 10.9 (Mavericks) calling the QFileDialog::getOpenFileName() method shows some files with a matching file extension greyed out. Even though com.apple.security.files.user-selected.read-write has been requested in the entitlements file and the filter parameter to getOpenFileName() is correct these files are greyed out when not in a folder that has a matching entitlement requested. However, files that are under the Music folder, for example, where com.apple.security.assets.music.read-only is also requested do allow files with the filtered file extension to be opened.
NB: A workaround to allow opening a greyed out file is to right click on the file and choose 'Quick Look' in the file open dialog. That file then becomes selected and can be opened.