diff --git a/cgit.c b/cgit.c
index 29e075d..04682be 100644
--- a/cgit.c
+++ b/cgit.c
@@ -37,10 +37,10 @@ static struct cgit_filter *new_filter(const char *cmd, filter_type filtertype)
 
 	switch (filtertype) {
 		case SOURCE:
+		case ABOUT:
 			extra_args = 1;
 			break;
 
-		case ABOUT:
 		case COMMIT:
 		default:
 			extra_args = 0;
diff --git a/cgitrc.5.txt b/cgitrc.5.txt
index 8a0a9c9..ea0bbe7 100644
--- a/cgitrc.5.txt
+++ b/cgitrc.5.txt
@@ -542,9 +542,11 @@ config files, e.g. "repo.desc" becomes "desc".
 FILTER API
 ----------
 about filter::
-	This filter is given no arguments. The about text that is to be
-	filtered is available on standard input and the filtered text is
-	expected on standard output.
+	This filter is given a single parameter: the filename of the source
+	file to filter. The filter can use the filename to determine (for
+	example) the type of syntax to follow when formatting the readme file.
+	The about text that is to be filtered is available on standard input
+	and the filtered text is expected on standard output.
 
 commit filter::
 	This filter is given no arguments. The commit message text that is to
diff --git a/ui-repolist.c b/ui-repolist.c
index 47ca997..2ab6e9e 100644
--- a/ui-repolist.c
+++ b/ui-repolist.c
@@ -332,9 +332,13 @@ void cgit_print_site_readme()
 {
 	if (!ctx.cfg.root_readme)
 		return;
-	if (ctx.cfg.about_filter)
+	if (ctx.cfg.about_filter) {
+		ctx.cfg.about_filter->argv[1] = ctx.cfg.root_readme;
 		cgit_open_filter(ctx.cfg.about_filter);
+	}
 	html_include(ctx.cfg.root_readme);
-	if (ctx.cfg.about_filter)
+	if (ctx.cfg.about_filter) {
 		cgit_close_filter(ctx.cfg.about_filter);
+		ctx.cfg.about_filter->argv[1] = NULL;
+	}
 }
diff --git a/ui-summary.c b/ui-summary.c
index abf914e..ffad4f2 100644
--- a/ui-summary.c
+++ b/ui-summary.c
@@ -98,6 +98,7 @@ void cgit_print_summary()
 void cgit_print_repo_readme(char *path)
 {
 	char *slash, *tmp, *colon, *ref;
+	int free_filename = 0;
 
 	if (!ctx.repo->readme || !(*ctx.repo->readme))
 		return;
@@ -134,6 +135,7 @@ void cgit_print_repo_readme(char *path)
 				return;
 			slash = colon;
 		}
+		free_filename = 1;
 		tmp = xmalloc(slash - ctx.repo->readme + 1 + strlen(path) + 1);
 		strncpy(tmp, ctx.repo->readme, slash - ctx.repo->readme + 1);
 		strcpy(tmp + (slash - ctx.repo->readme + 1), path);
@@ -144,13 +146,19 @@ void cgit_print_repo_readme(char *path)
 	 * filesystem, while applying the about-filter.
 	 */
 	html("<div id='summary'>");
-	if (ctx.repo->about_filter)
+	if (ctx.repo->about_filter) {
+		ctx.repo->about_filter->argv[1] = tmp;
 		cgit_open_filter(ctx.repo->about_filter);
+	}
 	if (ref)
 		cgit_print_file(tmp, ref);
 	else
 		html_include(tmp);
-	if (ctx.repo->about_filter)
+	if (ctx.repo->about_filter) {
 		cgit_close_filter(ctx.repo->about_filter);
+		ctx.repo->about_filter->argv[1] = NULL;
+	}
 	html("</div>");
+	if (free_filename)
+		free(tmp);
 }