diff --git a/docs/plugins/hooks.rst b/docs/plugins/hooks.rst
index 37d6911c..735ec793 100644
--- a/docs/plugins/hooks.rst
+++ b/docs/plugins/hooks.rst
@@ -192,3 +192,38 @@ octoprint.comm.transport.serial.factory
    :rtype: A serial instance implementing implementing the methods ``readline(...)``, ``write(...)``, ``close()`` and
        optionally ``baudrate`` and ``timeout`` attributes as described above.
 
+.. _sec-plugins-hook-filemanager-extensiontree:
+
+octoprint.filemanager.extension_tree
+------------------------------------
+
+.. py:function:: hook()
+
+   Return additional entries for the tree of accepted file extensions for uploading/handling by the file manager.
+
+   Should return a dictionary to merge with the existing extension tree, adding additional extension groups to
+   ``machinecode`` or ``model`` types.
+
+   **Example:**
+
+   The following handler would add a new file type "x3g" as accepted ``machinecode`` format, with extensions ``x3g``
+   and ``s3g``:
+
+   .. code-block:: python
+      :linenos:
+
+      def support_x3g_machinecode():
+          return dict(
+              machinecode=dict(
+                  x3g=["x3g", "s3g"]
+              )
+          )
+
+   .. note::
+
+      This will only add the supplied extensions to the extension tree, allowing the files to be uploaded and managed
+      through the file manager. Plugins will need to add further steps to ensure that the files will be processable
+      in the rest of the system (e.g. handling/preprocessing new machine code file types for printing etc)!
+
+   :return: The partial extension tree to merge with the full extension tree.
+   :rtype: dict
\ No newline at end of file
diff --git a/src/octoprint/filemanager/__init__.py b/src/octoprint/filemanager/__init__.py
index 068caa2e..04961254 100644
--- a/src/octoprint/filemanager/__init__.py
+++ b/src/octoprint/filemanager/__init__.py
@@ -9,6 +9,7 @@ import logging
 import os
 
 import octoprint.plugin
+import octoprint.util
 
 from octoprint.events import eventManager, Events
 
@@ -17,19 +18,35 @@ from .analysis import QueueEntry, AnalysisQueue
 from .storage import LocalFileStorage
 
 extensions = dict(
-	# extensions for 3d model files
-	model=dict(
-		stl=["stl"]
-	),
-	# extensions for printable machine code
-	machinecode=dict(
-		gcode=["gcode", "gco", "g"]
-	)
 )
 
+def full_extension_tree():
+	result = dict(
+		# extensions for 3d model files
+		model=dict(
+			stl=["stl"]
+		),
+		# extensions for printable machine code
+		machinecode=dict(
+			gcode=["gcode", "gco", "g"]
+		)
+	)
+
+	extension_tree_hooks = octoprint.plugin.plugin_manager().get_hooks("octoprint.filemanager.extension_tree")
+	for name, hook in extension_tree_hooks.items():
+		try:
+			hook_result = hook()
+			if hook_result is None or not isinstance(hook_result, dict):
+				continue
+			result = octoprint.util.dict_merge(result, hook_result)
+		except:
+			logging.getLogger(__name__).exception("Exception while retrieving additional extension tree entries from hook {name}".format(name=name))
+
+	return result
+
 def get_extensions(type, subtree=None):
 	if not subtree:
-		subtree = extensions
+		subtree = full_extension_tree()
 
 	for key, value in subtree.items():
 		if key == type:
@@ -43,7 +60,7 @@ def get_extensions(type, subtree=None):
 
 def get_all_extensions(subtree=None):
 	if not subtree:
-		subtree = extensions
+		subtree = full_extension_tree()
 
 	result = []
 	if isinstance(subtree, dict):
@@ -58,7 +75,7 @@ def get_all_extensions(subtree=None):
 
 def get_path_for_extension(extension, subtree=None):
 	if not subtree:
-		subtree = extensions
+		subtree = full_extension_tree()
 
 	for key, value in subtree.items():
 		if isinstance(value, (list, tuple)) and extension in value:
@@ -70,11 +87,9 @@ def get_path_for_extension(extension, subtree=None):
 
 	return None
 
-all_extensions = get_all_extensions()
-
 def valid_extension(extension, type=None):
 	if not type:
-		return extension in all_extensions
+		return extension in get_all_extensions()
 	else:
 		extensions = get_extensions(type)
 		if extensions:
diff --git a/src/octoprint/server/util/watchdog.py b/src/octoprint/server/util/watchdog.py
index a335be05..0ea409b4 100644
--- a/src/octoprint/server/util/watchdog.py
+++ b/src/octoprint/server/util/watchdog.py
@@ -17,10 +17,8 @@ class GcodeWatchdogHandler(watchdog.events.PatternMatchingEventHandler):
 	Takes care of automatically "uploading" files that get added to the watched folder.
 	"""
 
-	patterns = map(lambda x: "*.%s" % x, octoprint.filemanager.get_all_extensions())
-
 	def __init__(self, file_manager, printer):
-		watchdog.events.PatternMatchingEventHandler.__init__(self)
+		watchdog.events.PatternMatchingEventHandler.__init__(self, patterns=map(lambda x: "*.%s" % x, octoprint.filemanager.get_all_extensions()))
 
 		self._logger = logging.getLogger(__name__)
 
@@ -55,7 +53,7 @@ class GcodeWatchdogHandler(watchdog.events.PatternMatchingEventHandler):
 			futureFilename = self._file_manager.sanitize_name(octoprint.filemanager.FileDestinations.LOCAL, file_wrapper.filename)
 		except:
 			futureFilename = None
-		if futureFilename is None or (len(self._file_manager.registered_slicers) == 0 and not octoprint.filemanager.valid_file_type(futureFilename, type="gcode")):
+		if futureFilename is None or (len(self._file_manager.registered_slicers) == 0 and not octoprint.filemanager.valid_file_type(futureFilename)):
 			return
 
 		# prohibit overwriting currently selected file while it's being printed