Opera

Downloads and File Types

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.

Configuring settings

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.

Handling file types

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.

Potentially hazardous file types

Most of these file types can do no damage to Mac and UNIX operating systems. The known exception is macro viruses.

.exe

Files ending in this are executable files, applications unto themselves. Viruses of this type can do anything a regular application can do, including deleting files on your computer. Executable files will very rarely be used legitimately as attachments. Never run .exe files you do not know the contents of.

.pif

A PIF is a program information file that contains the necessary information for running DOS applications. Executable files can also be renamed to .pif and remain functional, which means that .pif files should be treated with at least the same caution as .exe files.

.com

This file type is less common, but it is also a form of executable file. Note that this applies to .com as a file extension only, and not to Web sites ending in ".com".

.vbs

These three letters are short for "Visual Basic Script". The infamous "I love you" virus was a VBS file. This type of virus requires that your computer be capable of running Visual Basic scripts. Most computers running Microsoft Windows are, while Linux users have no worries here.

.bat, .cmd

Batch file that runs DOS commands have the .bat extension. Files ending in .cmd are Windows NT script files. Both may, if infectious, cause harm to your computer system.

.doc, .xls, .ppt

These extensions, used by Microsoft Office documents (Word, Excel, and PowerPoint, respectively), can carry a special type of virus called a macro virus. The important thing to know is that macro viruses can live in such documents, and that they can do almost as much damage as viruses that end in "exe".

.rtf

RTF stands for "rich text format," and is quite common as a document format. Documents ending in ".rtf" can, however, be Microsoft Word documents in disguise (with the extension changed). The same precautions should therefore be taken for these documents as for any .doc-document.

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.