The official (or just well-known) formal name of the file type. Generally returned by Web servers to their client Web browsers. Non-registered MIME types typically start with string "application/x-".

A MIME type may end with a wildcard. Example: "image/*" matches "image/gif", "image/jpeg", etc.

Examples: text/plain, image/jpeg, application/excel, application/x-java-help-index.

If the filename or URL of a file ends with specified ".extension", then this file is detected as having this file type.

An extension may or may not start with a dot. This is unimportant because, in all cases, a leading dot would be automatically stripped.

Examples: txt, jpeg, jpg, xls.

For some file formats, the first bytes of a file are always the same and therefore, can be considered as being the signature of this file type.

If a file starts with specified first bytes, then this file is detected as having this file type. This type of detection is supposed to work like magic, hence the name: ``magic string''.

A magic string may be specified by a the hexadecimal representation of a sequence of bytes (example, one of the two TIFF magic strings: 4949) or by a quoted sequence of ASCII characters (same example, one of the two TIFF magic strings: "II").

Examples: TIFF: "II" or 4949, "MM"or 4D4D; GIF: "GIF87a", "GIF89a"; PNG: 89504E47; PDF: "%PDF-".

If the root element of an XML file has a name which matches specified pattern, then this XML file is detected as having this file type.

An XML name pattern follows this syntax:

( '{' namespace_URI? '}' )? local_part

One of local_part or namespace_URI may be equal to wildcard "*"

Examples: {*}svg, {http://www.w3.org/1998/Math/MathML}:math.

Description of the file type. Not mandatory, just recommended. This text is displayed in the File types list of the "Helper applications" preferences sheet.

One or more MIME types (see definition above) separated by spaces.

One or more extensions (see definition above) separated by spaces.

One or more magic strings (see definition above) separated by spaces.

One or more XML name patterns (see definition above) separated by spaces.

This field must contain a command line interpreted by the native shell of the platform: cmd.exe on Windows and /bin/sh on the Mac and on Unix.

This command line must reference one of these two substituted variables: %U and %F. In principle, %U is replaced by the URL of the file to be opened by the helper application and %F is replaced by a filename. In practice, %U is just a hint meaning: the helper application can open URLs as well as filenames.

The Choose Helper Application button displays the standard file chooser in order to specify an application (e.g. a .exe or a .bat file on Windows). String " "%F"" is automatically appended to the chosen application.

Helper applications on the Mac

When an application, that is, a folder having a name ending with hidden suffix ".app", containing a package bundle, has been selected by the user, the Choose Helper Application button automatically prepends "open -W -n -a " to chosen .app folder. Example: "open -W -n -a /Applications/Inkscape "%F"". Options "-W -n" mean: start a new instance of the application and wait until this instance has exited. These options are required when the helper application is used to edit the content of an element, the content of an attribute or the whole document.

Files having specified type are to be opened using the default viewer. The default viewer is specified in the "Helper applications" section of the "Preferences" dialog box.

Files having specified type are to be opened using the “default text editor”.

	Which text editor?
	This text editor to be used is the helper application associated to the "text/plain" MIME type. If such association has not been defined, the standard text editor of the platform is automatically used (e.g. on Windows, it's notepad).

Use XXE as an “internal” helper application to open files having specified type. For example, associating XXE to .mml files lets you edit the MathML documents referenced in a DocBook document.