Added S/MIME file type hint

app/src/main/java/eu/faircode/email/FragmentOptionsEncryption.java

@@ -325,6 +325,7 @@ public class FragmentOptionsEncryption extends FragmentBase implements SharedPre
         btnImportKey.setOnClickListener(new View.OnClickListener() {
             @Override
             public void onClick(View v) {
+                // https://pki-tutorial.readthedocs.io/en/latest/mime.html
                 Context context = v.getContext();
                 PackageManager pm = context.getPackageManager();
                 Intent open = new Intent(Intent.ACTION_GET_CONTENT);

app/src/main/res/layout/fragment_options_encryption.xml

@@ -355,6 +355,18 @@
                     app:layout_constraintStart_toStartOf="parent"
                     app:layout_constraintTop_toBottomOf="@id/btnManageCertificates" />
 
+                <eu.faircode.email.FixedTextView
+                    android:id="@+id/tvImportKeyHint"
+                    android:layout_width="0dp"
+                    android:layout_height="wrap_content"
+                    android:layout_marginEnd="48dp"
+                    android:text="*.p12, *.pfx"
+                    android:textAppearance="@style/TextAppearance.AppCompat.Small"
+                    android:textColor="?android:attr/textColorPrimary"
+                    app:layout_constraintEnd_toEndOf="parent"
+                    app:layout_constraintStart_toStartOf="parent"
+                    app:layout_constraintTop_toBottomOf="@id/btnImportKey" />
+
                 <Button
                     android:id="@+id/btnManageKeys"
                     style="?android:attr/buttonStyleSmall"
@@ -365,7 +377,7 @@
                     android:drawablePadding="6dp"
                     android:text="@string/title_advanced_manage_keys"
                     app:layout_constraintStart_toStartOf="parent"
-                    app:layout_constraintTop_toBottomOf="@id/btnImportKey" />
+                    app:layout_constraintTop_toBottomOf="@id/tvImportKeyHint" />
 
                 <eu.faircode.email.FixedTextView
                     android:id="@+id/tvManageKeysHint"