These extensions follow a period at the end of a filename in the format filename. These extensions define the file type so browsers and email programs can interpret the file correctly. Browsers can natively display files of certain types like HTML and sound. Sometimes, files require plug-ins like Adobe Acrobat Reader or QuickTime. Other files require helper applications like zip files and Word documents.
Mail programs use MIME types to read attachments.
Files on websites must have extensions to be interpreted properly by browsers. UNIX and Windows programs use extensions to define file types. Windows 95 and NT can hide file types from users. Macs use file types, but they are hidden from the user. A tool like ResEdit allows users to view and change Macintosh file types.