code: def openSourceFile(self, fileToOpen):
  		
  		if self.fileName:
  			currentExt = splitext(self.fileName)[1]
  			basename, ext = splitext(fileToOpen)
  			if ext in ('.html', '') and exists(basename + currentExt):
  				self.p.openFileWrapper(basename + currentExt)
  				return basename + currentExt
  		if exists(fileToOpen) and get_markup_for_file_name(fileToOpen, return_class=True):
  			self.p.openFileWrapper(fileToOpen)
  			return fileToOpen
docs: Finds and opens the source file for link target fileToOpen.
  
  		When links like [test](test) are clicked, the file test.md is opened.
  		It has to be located next to the current opened file.
  		Relative paths like [test](../test) or [test](folder/test) are also possible.