As this Ubuntu wiki says:
The file manager creates thumbnails to preview image and video files,
and can also preview text files and sound files......The icon for
plain text files looks like a piece of paper with text inside. The
file manager automatically extracts the first few lines of text from
the file and shows it in the icon. You can turn this feature on or
off, or turn it on only for files on your computer and local external
To turn off the feature:
nautilus and go to Edit -> Preferences.
Go to the Previews tab and under the Text Files section, select the menu for Show text in icons and select Never.
Close the Preferences.
Now, we come to changing icons.
I've Faenza Icon Theme installed, so I go to
/usr/share/icons/Faenza/mimetypes/scalable and there is already an icon file named
text-x-python.svg, and the same shows for all the
If you've any icon theme installed you should go to the directory for that theme and make the changes, if not you should change the icons in the
Additional Links: You can read more about File Manager Preferences here: https://help.ubuntu.com/11.04/ubuntu-help/nautilus-prefs.html.