This is in no way an exhaustive list, but should cover the most commonly seen file types for the typical home or office user.
*I took an exhaustive list from Wikipedia and pulled out just the most familiar ones. If anyone wants to add or suggest notes about danger levels for these file types I'd be happy to update it.
Microsoft Office Files
- DOC - Word '97-2003 or older document
- DOCX - Word 2007 or newer document
- XLS - Excel '97-2003 or older spreadsheet
- XLSX - Excel 2007 or newer spreadsheet
- PPT - Powerpoint '97-2003 or older presentation
- PPTX - Powerpoint 2007 or newer presentation
Other Document Types
- RTF - Rich-Text document (WordPad)
- TXT - Plain-Text document (Notepad)
- PDF - Portable Document Format (Adobe Reader or Acrobat)
- CSV - Comma-separated values (can be opened in Excel)
Image Files (Raster)
- JPG or JPEG
- TIF or TIFF
- ICO - Icon graphic
- OTF - OpenType Font
- TTF - TrueType Font
- PS - PostScript Font
Executable (auto-run) *These are the most dangerous file types if you're downloading from an unknown source. Always use caution when running these file!
- EXE - standard Windows executable
- JAR - archives of Java class files
- MSI - common Microsoft installer file
Music & Audio
- WAV - Microsoft Wave
- WMA - Windows Media Audio
- MP2/MP3 - MPEG Layer 2, Layer 3
- MPEG or MPG - MPEG-4
- MOV - QuickTime movie
- WMV - Windows Media Video
Compressed Files *These files wrap other groups of files and compress their data for easy download and sharing. Once extracted you will have a folder with one or more files inside. Compressed Files themselves are generally not dangerous, it's the files contained inside that you need to check and be aware of.
- ZIP - standard compressed file, use Windows compression/extraction or a software such as WinZip to open. (For most things the compression/extraction built into Windows XP/Vista/7 works just fine and there's little need to install any additional software.)
- 7z - 7-zip compressed file
- DMG - An Apple compressed/encrypted format
- JAR - ZIP for use with Java - make sure the source is trusted!
- RAR - Rar Archive
If you are downloading a file type that you are not familiar with and that is not listed above I highly recommend doing some research about the file type before opening or running it. Make sure the place or person you received the file from is known to you or has a good reputation. Look for comments from other people who have downloaded and used the file before you to learn whether anyone has had any trouble with their computer after using the file.