How to hide non-openable in the finder while using an application to browse/open For example, in the following picture. I am using TeXstudio to open a .tex file. After pressing Cmd-O, the files with extensions that are not in the extension drop-down menu ("TeX files (*.tex, *.bib, etc)") are gray items in the pop-up Finder window. 



My question is: How to hide these files? Is there a keyboard shortcut to do this?
No, there is no way to hide these files. It's a usability decision by Apple.

The dialog always shows the same list of files but greys out the ones you currently cannot open. Technically you could go to the pop up at the bottom and change it to All files and open any of the files listed. Of course they may not make sense in the application, but you could open them.