About this interactive
A file extension is the part of the name after the period, and it tells the operating system which program opens the file.
Word saves .doc in Word 2003 and earlier and .docx in Word 2007 and later; Excel does the same with .xls and .xlsx. A .txt file is text without formatting, opened in Notepad on Windows; .rtf, rich text format, is text with formatting. One program can save several formats: Paint's Save as offers PNG, JPEG, BMP and GIF, and a bitmap is saved uncompressed while a JPEG (.jpg) is compressed. One app can own several extensions too: Chrome opens both .htm and .html, and Adobe Acrobat opens .pdf.
Type the extension without its dot. Capitals do not matter.
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