Allow import armored (PEM) S/MIME private key from file

[rrrooommmaaa]

Currently , we can import an S/MIME key (during initial setup)
either with

• "Load from file" option, providing `test/samples/smime/human-unprotected-PKCS12.p12"
• "Paste armored key directly". pasting the armored key and certificate from "test/samples/smime/human-unprotected-pem.txt"

However, when I try to use "Load from file" option with "test/samples/smime/human-unprotected-pem.txt" (under Windows, if it matters), the screen remains silent (without any error messages and not passing to the next screen)