extension and their types

A filename extension is a suffix (separated from the base filename by a dot or space) to the name of a computer file applied to indicate the encoding (file format) of its contents or usage. Examples of filename extensions are .png, .jpeg, .exe, .dmg and .txt.
Some file systems limit the length of the extension (such as the FAT file system from PC DOS/MS-DOS (without Long filename support) not allowing more than three characters and IBM's VM/CMS not allowing more than 8) while others (such as NTFS) do not. Unix filesystems accept the separator dot as a legal character.

Filename extensions can be considered a type of metadata. They are commonly used to imply information about the way data might be stored in the file. The exact definition, giving the criteria for deciding what part of the file name is its extension, belongs to the rules of the specific filesystem used; usually the extension is the substring which follows the last occurrence, if any, of the dot character (example: txt is the extension of the filenamereadme.txt, and html the extension of mysite.index.html). On file systems of mainframe systems such as MVS, VMS, and PC systems such as CP/M and derivative systems such as MS-DOS, the extension is a separate namespace from the filename. Under Microsoft's DOS and Windows, extensions such as EXE, COM or BAT indicate that a file is a program executable.

Metadata (metacontent) are defined as the data providing information about one or more aspects of the data, such as:

·         Means of creation of the data

·         Purpose of the data

·         Time and date of creation

·         Creator or author of the data

·         Location on a computer network where the data were created

·         Standards used

For example, a digital image may include metadata that describe how large the picture is, the color depth, the image resolution, when the image was created, and other data.^[2] A text document's metadata may contain information about how long the document is, who the author is, when the document was written, and a short summary of the document.