Opera handles a large number of varying file types, including HTML files, graphical files such as JPEG and GIF, and other types of files which it cannot use by itself. These include PDF documents, word-processor documents which can end in SXW, RTF, DOC, or any of a dozen other extensions. For certain file types, Opera needs multimedia plug-ins.
By default, Opera will determine how to handle a file by its MIME type. MIME types are descriptions used by Web servers to identify files to browsers. This is the most secure way of receiving content on the Internet. There is a second option, however; you may choose to let Opera determine use the file's extension to decide which action to take when the MIME type is not reliable. This option is less secure than the default.
Some MIME types are intended as generic types, such as "text/plain" and "application/octet-stream". If a server is not specifically set up to handle a certain kind of content, these generic MIME types are often used. This means that sometimes a video file in an MPEG format will be sent using the "text/plain"MIME type. If you have chosen to determine action by file extension, Opera will nevertheless recognize the video file's extension (such as ".mpg"), and handle it according to your settings for .mpg files.
However, sometimes the file type indicated by the extension is not the file type that the browser interprets. This is due to an HTTP header called "content-disposition," which can assign a new name to the file you are downloading. Therefore, if you enable the option to determine file type by extension, pay close attention to the file name in "Open" and "Save" dialog boxes and make certain the file is not of a different kind than expected. If it is, do not open or run the file.
A sensible basic rule is not to let Opera open any file types that the browser itself cannot handle. Keep in mind that letting Opera launch files in other programs automatically makes the browser as insecure as the least secure of the other programs that you use. In other words, configuring Opera to automatically open ".doc" files in Microsoft Word makes your computer vulnerable to macro viruses that can run in Word documents and cause great damage to the computer system.
Note: When viruses attach to other files, they will often add their own extension after the original file extension and thus try to masquerade as a different file type. Before opening downloaded or received files, make sure they only have one, reliable extension.
Most of these file types can do no damage to Mac and UNIX operating systems. The known exception is macro viruses.
Note: If you are not familiar with the extension of a file you receive and are not certain which program it will open in and whether it may cause harm, do some research before deciding what to do with it. For an overview of file extensions on Windows, Mac and UNIX, try file-ext.com.