“The application “%@” can’t be opened.” However doing “open with” and selecting default app works

I got QNapi to show up in the open with menu by changing

<key>CFBundleTypeRole</key>
<string>None</string>

to

<key>CFBundleTypeRole</key>
<string>Viewer</string>

and then running /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -f /Applications/QNapi.app and quitting and reopening Finder.

(lsregister -f forces a file to be registered even if its modification date has not changed.)

In Yosemite, I can open a file with any application in the Dock by dragging the file's icon over the application while holding the command and option keys down. This only works if the targeted application is indeed able to open the file, of course.

Another option, when the application is not currently in your Dock, is to click the file with the right mouse button and choose an application from the submenu Open with, which is the second item in the context menu. This submenu lists the applications that can open the file you clicked on, with the default one at the top.