ui_plain: automatically lookup mimetype when mimetype-file is set

For sites that do not want to configure mime types by hand but
still want the correct mime type for 'plain' blobs, configuring
a mime type file is made possible. This is handy since such a
file is normally already provided (at least on Linux systems).

Also, this reflects the gitweb option '$mimetypes_file'

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgitrc.5.txt

@@ -226,6 +226,17 @@ mimetype.<ext>::
 	Set the mimetype for the specified filename extension. This is used
 	by the `plain` command when returning blob content.
 
+mimetype-file::
+	Specifies the file to use for automatic mimetype lookup. If specified
+	then this field is used as a fallback when no "mimetype.<ext>" match is
+	found. If unspecified then no such lookup is performed. The typical file
+	to use on a Linux system is /etc/mime.types. Default value: none. See
+	also: "mimetype.<ext>".	The format of the file must comply to:
+	- a comment line is an empty line or a line starting with a hash (#),
+	  optionally preceded by whitespace
+	- a non-comment line starts with the mimetype (like image/png), followed
+	  by one or more file extensions (like jpg), all separated by whitespace
+
 module-link::
 	Text which will be used as the formatstring for a hyperlink when a
 	submodule is printed in a directory listing. The arguments for the