class_docstr: Determines the extension of a filepath.
  
  Description:
      Given a list of filepaths, return the extension
      suffix of each one. If the filepath is missing
      or invalid, return `NaN`.
  
  Examples:
      >>> file_extension = FileExtension()
      >>> file_extension(['doc.txt', '~/documents/data.json', 'file']).tolist()
      ['.txt', '.json', nan]
class_docstr_tok: ["Determines","the","extension","of","a","filepath",".","Description",":","Given","a","list","of","filepaths",",","return","the","extension","suffix","of","each","one",".","If","the","filepath","is","missing","or","invalid",",","return","`","NaN","`",".","Examples",":",">",">",">","file_extension","=","FileExtension","(",")",">",">",">","file_extension","(","[","'doc.txt","'",",","'~\/documents\/data.json","'",",","'file","'","]",")",".tolist","(",")","[","'.txt","'",",","'.json","'",",","nan","]"]
code: def get_function(self):
      def file_extension(x):
          p = r"(\.[a-z|A-Z]+$)"
          return x.str.extract(p, expand=False).str.lower()
  
      return file_extension
  
imports: from woodwork.column_schema import ColumnSchema
  from woodwork.logical_types import Filepath
  from featuretools.primitives.base import TransformPrimitive
label_desc: Use image node_id 1 for calling the FileExtension obj's underlying member method code with example usage: obj.get_function() and returns: file_extension, x