I can't find the "Open with..." option for Pencil documents.
A ".ep" file is a saved Pencil document.
How can I set the default application to open ".ep" files since there is no "Open with..." option?
This is a 'Pencil' document, specifically a saved wireframe. For those who don't know what Pencil is, please see here.
Sadly, Pencil does not allow you to register file extension types (and open them correctly). This is due to it being built on Firefox technology. This means that it isn't recognised as an application, but as an extension of the original software. I've had this issue on Ubuntu and Windows both (I use Balsamiq on Mac). I've looked into many avenues for registering extensions, and none of them work.
Sorry I couldn't be more help.
1) first you need a pencil.desktop file in /usr/share/applications/
put this content into the file:
[Desktop Entry]
Encoding=UTF-8
Name=Pencil
Comment=Sketching and GUI prototyping tool
Comment[vi_VN]=Công cụ phát thảo giao diện Pencil
Exec=/usr/bin/pencil %U
Terminal=false
Type=Application
StartupNotify=true
Icon=/usr/share/pencil/skin/classic/icon.svg
Categories=Graphics;2DGraphics;Development;
MimeType=application/ep;
2) then change the pencil file in /usr/bin/
#!/bin/sh
#
xulrunner --app "/usr/share/pencil/application.ini" "$1"
the important part is the argument variable $1 after the last line
3) profit