Added documentation section for ftp custom converter functionality

docs/iot-gateway/config/ftp.md

@@ -193,7 +193,7 @@ Let's look at how we can configure this section for different file extensions:
         }
       ]
     ```
-    
+
     That means that FTP converter will look for the following file structure:
     ```txt
     temp,hum
@@ -312,6 +312,98 @@ Let's look at how we can configure this section for different file extensions:
         ]
   ```
 
+#### Subsection "converter"
+
+This configuration section is **optional** and is used only when you want to process incoming data with a custom FTP converter.
+
+Typical use cases:
+
+- Files with **extensions not supported** by default, or supported extensions, but with a different data structure, for example:
+  - CSV files without a header line
+  - CSV lines where fields are located at fixed column positions
+
+In this example, we show a configuration for a **CSV file without headers**, where device name, attributes and telemetry values are taken by column index.
+
+Mapping subsection in the configuration looks like:  
+
+```json
+{
+  "converter": {
+    "type": "custom",
+    "extension": "CustomFTPtUplinkConverter",
+    "extension-config": {
+      "devicePatternName": 0,
+      "devicePatternType": 1,
+      "attributes": [
+        {
+          "key": "meterAddress",
+          "column": 0,
+          "type": "string"
+        }
+      ],
+      "timeseries": [
+        {
+          "key": "meterReading",
+          "column": 2,
+          "type": "double"
+        },
+        {
+          "key": "batteryAlert",
+          "column": 3,
+          "type": "int"
+        }
+      ]
+    }
+  }
+}
+```
+{:.copy-code}
+
+Below are description of each parameter used in this configuration for the custom converter.
+
+Top-level converter parameters:
+
+1. **converter.type**  
+   Must be set to **custom**. This tells the FTP connector to use a custom converter instead of the built-in ones.
+
+2. **converter.extension**  
+   Name of the Python class that implements the custom converter, for example **CustomFTPtUplinkConverter**.
+
+
+Extension-config fields:
+
+1. **devicePatternName**  
+   Column index used to resolve the device name. In the example, **0** means that the device name is taken from column 0.
+
+2. **devicePatternType**  
+   Column index or literal used to resolve the device type. In the example, **1** means that the device type is taken from column 1.
+
+3. **attributes**  
+   An array of attribute mappings. Each item describes how to read a single device attribute from a specific CSV column.
+
+4. **timeseries**  
+   An array of telemetry mappings. Each item describes how to read a single telemetry value from a specific CSV column.
+
+
+Attributes and timeseries items:
+Each element of attributes and timeseries has the same structure:
+
+1. **key**  
+   Attribute or telemetry key that will appear on the created device in the ThingsBoard Platform (for example, **meterAddress**, **meterReading**).
+
+2. **column**  
+   Zero-based column index in the CSV line from which the value is taken.
+
+3. **type**  
+   Target value type. The custom converter will cast the raw string to this type (for example, **string**, **int**, **double**).
+
+{% capture difference %}
+The exact behavior will depend on your own `CustomFTPtUplinkConverter` implementation, but this configuration shows a working example for headerless CSV files.
+You will most likely change the mapping values (devicePatternName, devicePatternType, attributes, timeseries) to match your file format, but the parameter names and structure must remain the same.
+{% endcapture %}
+{% include templates/info-banner.md content=difference %}
+
+
 ### Section "attributeUpdates"