I would just like to know what I have to put in the Resources file to activate that key combination.
This:
<MJRS version="1.0">
<ACCELERATORS>
<!-- Opens the "File>Open media file" dialogue (replaces the default "Select All" command -->
<Entry Key="Ctrl;A" Command="20000" />
</ACCELERATORS>
</MJRS>
Remembering that you need to create a new Resource.xml file in a "Custom Resources" folder as per instructions linked to in my first reply. If you do not do this, subsequent MC updates will remove any additions you make to the default file.
I have tested this here, and it works as I think you want it to, as in, it opens that dialogue. Mine defaulted to opening in the MC program folder.
I've attached a copy of my own resource file in case you are interested. As well as the Ctrl+a you required (which does replace the existing "Select All" command), it has:
Ctrl+T to toggle the tree open/closed
Ctrl+Alt+D to open explorer at the folder of the selected file, and select that file in said folder.
Ctrl+Alt+R to start a seedless play doctor session (this one is global)