Add a toolbar button to open a file

[KrasnayaPloshchad]

This button would allow users opening fb2 file locally, as EPUBReader did.

[tymofij]

How would it be different than just using menu item File → Open File… ?

[KrasnayaPloshchad]

With menu item File → Open File… the plain fb2 file works, however fb2.zip ceased to work.