OS X Yosemite: Show or hide filename extensions
Filename extensions—the period followed by a few letters or words that you see at the end of some filenames (for example, .jpg)—indicate which app can open a file.
Filename extensions are usually hidden in OS X, but if you find them useful, you can show them. If extensions are hidden, OS X still opens files with the proper apps.
For one file
Select a file, then choose File > Get Info, or press Command (⌘)–I.
Click the triangle next to Name & Extension to expand the section.
To show or hide the filename extension, select or deselect “Hide extension.”
For all files
If you select “Show all filename extensions,” all extensions are shown, even for files that have “Hide extension” selected. If you deselect “Show all filename extensions,” then file extensions are shown or hidden based on their individual “Hide extension” settings.
Choose Finder > Preferences, then click Advanced.
Select or deselect “Show all filename extensions.”
When you rename a file or folder, don’t change its filename extension. Or, you may no longer be able to open the file with the app that was used to create it. To be warned before you change an extension, select “Show warning before changing an extension” in the Advanced pane of Finder preferences.
If you want to change a file’s format, use the app you used to create the file. For example, TextEdit can convert a document from plain text format (.txt) to a rich text format (.rtf), and Preview can convert many types of graphics files.