* doc/lispref/files.texi (File Attributes): Document it.
* lisp/files.el (file-has-changed-p): New function.
(file-has-changed-p--hash-table): Internal variable used by the
new function (bug#51523).
* lisp/emacs-lisp/shortdoc.el (file): Mention it.
* lisp/net/mailcap.el (mailcap-parse-mailcaps): Read mailcaps again
only when at least one of the mailcap files has changed. Fixes
bug#51523.