I was perusing doc-view.el and I've been quite puzzled by doc-view-set-doc-type.
(defun doc-view-set-doc-type ()
"Figure out the current document type (`doc-view-doc-type')."
(let ((name-types
(when buffer-file-name
(cdr (assoc-string
(file-name-extension buffer-file-name)
'(
;; DVI
("dvi" dvi)
;; PDF
("pdf" pdf) ("epdf" pdf)
;; PostScript
("ps" ps) ("eps" ps)
;; DjVu
("djvu" djvu)
;; OpenDocument formats.
("odt" odf) ("ods" odf) ("odp" odf) ("odg" odf)
("odc" odf) ("odi" odf) ("odm" odf) ("ott" odf)
("ots" odf) ("otp" odf) ("otg" odf)
;; Microsoft Office formats (also handled by the odf
;; conversion chain).
("doc" odf) ("docx" odf) ("xls" odf) ("xlsx" odf)
("ppt" odf) ("pps" odf) ("pptx" odf) ("rtf" odf))
t))))
(content-types
(save-excursion
(goto-char (point-min))
(cond
((looking-at "%!") '(ps))
((looking-at "%PDF") '(pdf))
((looking-at "\367\002") '(dvi))
((looking-at "AT&TFORM") '(djvu))))))
(setq-local
doc-view-doc-type
(car (or (doc-view-intersection name-types content-types)
(when (and name-types content-types)
(error "Conflicting types: name says %s but content says %s"
name-types content-types))
name-types content-types
(error "Cannot determine the document type"))))))
It is written as if, for a given document (in a buffer), there could be more than a name type (but name-types is derived from the file's extension), and more than one content type (but content-types is derived from looking at the few first bytes at the beginning of the buffer). And then there's a function doc-view-intersection whose only use is to detect cases of the name-types in the content-types' "list".
I mean, I could see the logic if it the code tried to detect cases of one extension (let's say, ".doc") that could represent different content-types. But in fact, currently the situation is the opposite: multiple extensions associated to one content-type (like .ps / .eps, etc.)
Seems to me generality for generality's sake, as the code has only had minor alteration (adding djvu and some assoc changes) in nine years.