code: def getExtensionList(self,extensions):
          
          basicList = extensions.split(',')
          extensionList = []
          for ext in basicList:
              if ext == MapConstants.placeholderNoExtensionFilter:
                  # Files without an extension are permitted:
                  extensionList.append('')
              elif ext != '':
                  # The '.' is prepended if ext does not start with '.' already:
                  extWithDot = ext if ext.startswith('.') else '.'+ext
                  extensionList.append(extWithDot)
          return list(set(extensionList))
docs: This is an internal method that transforms the comma-separated extensions string
          into a list of extensions, e.g., "ext1,ext2,ext3" gets turned into ['.ext1','.ext2','.ext3'].
          If MapConstants.placeholderNoExtensionFilter is part of the string, the resulting list
          will also contain '', i.e., files without extensions are permitted.