How to filter multiple file-extensions to limit which files display in an operator's file-selector? I'm creating an image file importer based off the operator_file_import template. I want to set script so that it recognizes image files, because right now it's only recognizing text. How do I go about defining a list of extensions that show up in the file browser when I launch the script? Right now, based on the starting template, the only thing I can import is a .txt file.

Here is the code - The generic comments are still there and anything that I changed is indicated by a comment/CHANGED tag. 

import bpy
from bpy.types import Operator
import collections
import os
from bpy.props import (
        StringProperty,
        BoolProperty,
        EnumProperty,
        IntProperty,
        FloatProperty,
        CollectionProperty,
        )
from bpy_extras.io_utils import ImportHelper
from bpy.types import Operator


#CHANGED read definition to class name
def read_pbr_textures(context, filepath, use_some_setting):   
    print("running read_pbr_textures..")
    f = open(filepath, 'r', encoding='utf-8')
    data = f.read()
    f.close()

    # would normally load the data here
    print(data)

    return {'FINISHED'}


#CHANGED class name
class ImportPbrTextures(Operator, ImportHelper):   
    """This appears in the tooltip of the operator and in the generated docs"""
    bl_idname = "import_test.pbr_textures"  # important since its how bpy.ops.import_test.some_data is constructed
    bl_label = "Import PBR Textures"

#CHANGED filename_ext to png
    # ImportHelper mixin class uses this
    filename_ext = "*.png"

    filter_glob = StringProperty(
            default="*.png",
            options={'HIDDEN'},
            maxlen=255,  # Max internal buffer length, longer would be clamped.
            )

    # List of operator properties, the attributes will be assigned
    # to the class instance from the operator settings before calling.
    use_setting = BoolProperty(
            name="Example Boolean",
            description="Example Tooltip",
            default=True,
            )

    type = EnumProperty(
            name="Example Enum",
            description="Choose between two items",
            items=(('OPT_A', "First Option", "Description one"),
                   ('OPT_B', "Second Option", "Description two")),
            default='OPT_A',
            )

    def execute(self, context):
        return read_some_data(context, self.filepath, self.use_setting)


# Only needed if you want to add into a dynamic menu
def menu_func_import(self, context):
    self.layout.operator(ImportPbrTextures.bl_idname, text="Text Import Operator")

#CHANGED class registration to ImportPbrTextures class
def register():
    bpy.utils.register_class(ImportPbrTextures)
    bpy.types.INFO_MT_file_import.append(menu_func_import)


def unregister():
    bpy.utils.unregister_class(ImportPbrTextures)
    bpy.types.INFO_MT_file_import.remove(menu_func_import)


if __name__ == "__main__":
    register()

    # test call
    bpy.ops.import_test.pbr_textures('INVOKE_DEFAULT')
In the operator's filter_glob property, (which you've already defined), you can define multiple extensions using the ; separator, eg:

filter_glob = StringProperty(
        default="*.png;*.jpg;*.jpeg;*.bmp;*.exr",
        options={'HIDDEN'},
        )


If you want to make a list of all extensions Blender supports you can use. bpy.path.extensions_image eg:

"*" + ";*".join(bpy.path.extensions_image)