Définition du paramétrage du pattern et mise en place des valeurs par défaut

etc/adminhtml/system.xml

+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
+    <system>
+        <section id="importimagesproduct" translate="label" sortOrder="15" showInDefault="1" showInWebsite="1" showInStore="1">
+            <class>separator-top</class>
+            <label>Import Images Product</label>
+            <tab>nicolasbejean</tab>
+            <resource>NicolasBejean_Importer::base</resource>
+            <group id="pattern" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
+                <label>File Pattern</label>
+                <field id="delimiter" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="1" showInStore="1">
+                    <label>Delimiter</label>
+                    <comment>Default value : "_"</comment>
+                </field>
+            </group>
+        </section>
+    </system>
+</config>

etc/config.xml

+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
+    <default>
+        <importimagesproduct>
+            <pattern>
+                <delimiter>_</delimiter>
+            </pattern>
+        </importimagesproduct>
+    </default>
+</config>