- added missing FileHandler docs

doc/input/filehandler.rst

@@ -0,0 +1,30 @@
+.. _filehandler:
+
+FileHandler
+===========
+
+Uploading files via HTTP requests can be configured by setting up the
+``FileHandler`` within the input handler configuration.
+
+.. code-block:: ini
+
+    [your-config]
+    endpoint = /upload
+    maxsize = 5000
+    type = FileHandler
+    folder.out = /app/data
+    file.prefix = out-
+    file.suffix = .csv
+
+folder.out
+    The directory to use for storing the file.
+
+file.prefix
+    File names are created randomly by the ``FileHandler``. To allow other
+    software to automatically recognize files provided by this specific
+    instance, a fixed prefix can be added to the random part.
+
+file.suffix
+    File names are created randomly by the ``FileHandler``. To allow other
+    software to automatically recognize files provided by this specific
+    instance, a fixed suffix can be added to the random part.