As this is emacs, the hardest part of doing what you want is deciding on what is the best approach. This largely depends on personal taste/workflows. I would highly recommend looking at the browse-url package in more detail. One thing I use is a function which allows me to switch between using eww or my default system browser - this means I can easily render web content either in emacs or in chrome/safari/whatever.
Some years ago, I wrote a utility which would allow me to view a number of different file formats, including rendered html, in emacs. I rarely use this now as doc-view has pretty much replaced most of this functionality and is much better. However, it does show how you can use defadvice to modify the view-file function so that id does different things depending on the file type. Note that as this is old emacs code and emacs has improved, there are probably better ways of doing this now. I also know that the 'advice' stuff has been re-worked, but this legacy stuff still works OK. Should get you started. Note that the functionality for MS doc, docx, pdf etc relies on external executables.
My preferred workflow would be to write a function which allows me to reset the browse-url-browser-function to either eww-browse-url or browse-url-default-browser and bind that to a key. I can then choose to display the html in emacs or the external browser and leverage of all the work already done in browse-url.
(require 'custom)
(require 'browse-url)
;; make-temp-file is part of apel prior to emacs 22
;;(static-when (= emacs-major-version 21)
;; (require 'poe))
(defgroup txutils nil
"Customize group for txutils."
:prefix "txutils-"
:group 'External)
(defcustom txutils-convert-alist
'( ;; MS Word
("\\.\\(?:DOC\\|doc\\)$" doc "/usr/bin/wvText" nil nil nil nil nil)
;; PDF
("\\.\\(?:PDF\\|pdf\\)$" pdf "/usr/bin/pdftotext" nil nil nil nil nil)
;; PostScript
("\\.\\(?:PS\\|ps\\)$" ps "/usr/bin/pstotext" "-output" t nil nil nil)
;; MS PowerPoint
("\\.\\(?:PPT\\|ppt\\)$" ppt "/usr/bin/ppthtml" nil nil nil t t))
"*Association for program convertion.
Each element has the following form:
(REGEXP SYMBOL CONVERTER SWITCHES INVERT REDIRECT-INPUT REDIRECT-OUTPUT HTML-OUTPUT)
Where:
REGEXP is a regexp to match file type to convert.
SYMBOL is a symbol to designate the fyle type.
CONVERTER is a program to convert the fyle type to text or HTML.
SWITCHES is a string which gives command line switches for the conversion
program. Nil means there are no switches needed.
INVERT indicates if input and output program option is to be
inverted or not. Non-nil means to invert, that is, output
option first then input option. Nil means do not invert,
that is, input option first then output option.
REDIRECT-INPUT indicates to use < to direct input from the input
file. This is useful for utilities which accept input
from stdin rather than a file.
REDIRECT-OUTPUT indicates to use > to direct output to the output
file. This is useful for utilities that only send output to
stdout.
HTML-OUTPUT Indicates the conversion program creates HTML output
rather than plain text."
:type '(repeat
(list :tag "Convertion"
(regexp :tag "File Type Regexp")
(symbol :tag "File Type Symbol")
(string :tag "Converter")
(choice :menu-tag "Output Option"
:tag "Output Option"
(const :tag "None" nil)
string)
(boolean :tag "Invert I/O Option")
(boolean :tag "Redirect Standard Input")
(boolean :tag "Redirect Standard Output")
(boolean :tag "HTML Output")))
:group 'txutils)
(defun txutils-run-command (cmd &optional output-buffer)
"Execute shell command with arguments, putting output in buffer."
(= 0 (shell-command cmd (if output-buffer
output-buffer
"*txutils-output*")
(if output-buffer
"*txutils-output*"))))
(defun txutils-quote-expand-file-name (file-name)
"Expand file name and quote special chars if required."
(shell-quote-argument (expand-file-name file-name)))
(defun txutils-file-alist (file-name)
"Return alist associated with file of this type."
(let ((al txutils-convert-alist))
(while (and al
(not (string-match (caar al) file-name)))
(setq al (cdr al)))
(if al
(cdar al)
nil)))
(defun txutils-make-temp-name (orig-name type-alist)
"Create a temp file name from original file name"
(make-temp-file (file-name-sans-extension
(file-name-nondirectory orig-name)) nil
(if (nth 7 type-alist)
".html"
".txt")))
(defun txutils-build-cmd (input-file output-file type-alist)
"Create the command string from conversion alist."
(let ((f1 (if (nth 3 type-alist)
output-file
input-file))
(f2 (if (nth 3 type-alist)
input-file
output-file)))
(concat
(nth 1 type-alist)
(if (nth 2 type-alist) ; Add cmd line switches
(concat " " (nth 2 type-alist)))
(if (nth 4 type-alist) ; redirect input (which may be output
(concat " < " f1) ; if arguments are inverted!)
(concat " " f1))
(if (nth 5 type-alist) ; redirect output (see above comment)
(concat " > " f2)
(concat " " f2)))))
(defun txutils-do-file-conversion (file-name)
"Based on file extension, convert file to text. Return name of text file"
(interactive "fFile to convert: ")
(let ((f-alist (txutils-file-alist file-name))
output-file)
(when f-alist
(message "Performing file conversion for %s." file-name)
(setq output-file (txutils-make-temp-name file-name f-alist))
(message "Command: %s" (txutils-build-cmd file-name output-file f-alist))
(if (txutils-run-command
(txutils-build-cmd (txutils-quote-expand-file-name file-name)
(txutils-quote-expand-file-name
output-file) f-alist))
output-file
file-name))))
(defadvice view-file (around txutils pre act comp)
"Perform file conversion or call web browser to view contents of file."
(let ((file-arg (ad-get-arg 0)))
(if (txutils-file-alist file-arg)
(ad-set-arg 0 (txutils-do-file-conversion file-arg)))
(if (string-match "\\.\\(?:HTML?\\|html?\\)$" (ad-get-arg 0))
(browse-url-of-file (ad-get-arg 0))
ad-do-it)))
(provide 'init-text-convert)
