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(elisp)MS-DOS File Types

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MS-DOS File Types
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   On MS-DOS and Microsoft Windows, Emacs guesses the appropriate
end-of-line conversion for a file by looking at the file's name.  This
feature classifies files as "text files" and "binary files".  By
"binary file" we mean a file of literal byte values that are not
necessarily meant to be characters; Emacs does no end-of-line conversion
and no character code conversion for them.  On the other hand, the bytes
in a text file are intended to represent characters; when you create a
new file whose name implies that it is a text file, Emacs uses DOS
end-of-line conversion.

 - Variable: buffer-file-type
     This variable, automatically buffer-local in each buffer, records
     the file type of the buffer's visited file.  When a buffer does
     not specify a coding system with `buffer-file-coding-system', this
     variable is used to determine which coding system to use when
     writing the contents of the buffer.  It should be `nil' for text,
     `t' for binary.  If it is `t', the coding system is
     `no-conversion'.  Otherwise, `undecided-dos' is used.

     Normally this variable is set by visiting a file; it is set to
     `nil' if the file was visited without any actual conversion.

 - User Option: file-name-buffer-file-type-alist
     This variable holds an alist for recognizing text and binary files.
     Each element has the form (REGEXP . TYPE), where REGEXP is matched
     against the file name, and TYPE may be `nil' for text, `t' for
     binary, or a function to call to compute which.  If it is a
     function, then it is called with a single argument (the file name)
     and should return `t' or `nil'.

     When running on MS-DOS or MS-Windows, Emacs checks this alist to
     decide which coding system to use when reading a file.  For a text
     file, `undecided-dos' is used.  For a binary file, `no-conversion'
     is used.

     If no element in this alist matches a given file name, then
     `default-buffer-file-type' says how to treat the file.

 - User Option: default-buffer-file-type
     This variable says how to handle files for which
     `file-name-buffer-file-type-alist' says nothing about the type.

     If this variable is non-`nil', then these files are treated as
     binary: the coding system `no-conversion' is used.  Otherwise,
     nothing special is done for them--the coding system is deduced
     solely from the file contents, in the usual Emacs fashion.