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>

cgit.c

@@ -236,6 +236,8 @@ void config_cb(const char *name, const char *value)
 		ctx.cfg.ssdiff = atoi(value);
 	else if (!strcmp(name, "agefile"))
 		ctx.cfg.agefile = xstrdup(value);
+	else if (!strcmp(name, "mimetype-file"))
+		ctx.cfg.mimetype_file = xstrdup(value);
 	else if (!strcmp(name, "renamelimit"))
 		ctx.cfg.renamelimit = atoi(value);
 	else if (!strcmp(name, "remove-suffix"))