eid-viewer: associate with XML & plain text .eid files

The .eid files are in fact XML, but lack the <?xml...?> declaration that
file(1) seems to expect. At least the viewer now appears as an option in
most GUIs.

pkgs/tools/security/eid-viewer/default.nix

@@ -21,6 +21,10 @@ stdenv.mkDerivation rec {
 
   postInstall = ''
     wrapProgram $out/bin/eid-viewer --suffix LD_LIBRARY_PATH : ${pcsclite}/lib
+    cat >> $out/share/applications/eid-viewer.desktop << EOF
+    # eid-viewer creates XML without a header, making it "plain text":
+    MimeType=application/xml;text/xml;text/plain
+    EOF
   '';
 
   meta = with stdenv.lib; {