diff --git a/src/PVE/GuestImport/OVF.pm b/src/PVE/GuestImport/OVF.pm
index 7dc4b24..1fae69b 100644
--- a/src/PVE/GuestImport/OVF.pm
+++ b/src/PVE/GuestImport/OVF.pm
@@ -354,7 +354,7 @@ ovf:Item[rasd:InstanceID='%s']/rasd:ResourceType", $controller_id);
 	}
 
 	if (!$isOva) {
-	    my $size = PVE::Storage::file_size_info($backing_file_path);
+	    my $size = PVE::Storage::file_size_info($backing_file_path, undef, 'auto-detect');
 	    die "error parsing $backing_file_path, cannot determine file size\n"
 		if !$size;
 
diff --git a/src/PVE/Storage.pm b/src/PVE/Storage.pm
index c8e1f2a..df8e948 100755
--- a/src/PVE/Storage.pm
+++ b/src/PVE/Storage.pm
@@ -2223,7 +2223,7 @@ sub assert_iso_content {
     my ($path) = @_;
 
     # check for things like backing image
-    file_size_info($path, undef, undef, 1);
+    file_size_info($path, undef, 'auto-detect', 1);
 
     return 1;
 }
diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
index 1f9156f..917bd61 100644
--- a/src/PVE/Storage/Plugin.pm
+++ b/src/PVE/Storage/Plugin.pm
@@ -959,10 +959,15 @@ my @checked_qemu_img_formats = qw(raw cow qcow qcow2 qed vmdk cloop);
 # - backing files (qcow2/vmdk)
 # - external data files (qcow2)
 #
-# Set $file_format to force qemu-img to treat the image as being a specific format.
+# Set $file_format to force qemu-img to treat the image as being a specific format. Use the value
+# 'auto-detect' for auto-detection. The parameter is planned to become mandatory with Proxmox VE 9.
 sub file_size_info {
     my ($filename, $timeout, $file_format, $untrusted) = @_;
 
+    # TODO PVE 9 make $file_format mandatory
+    warn "file_size_info: detected call without \$file_format parameter\n"
+	if !defined($file_format);
+
     # compat for old parameter order
     # TODO PVE 9 remove
     if (defined($file_format) && ($file_format eq '1' || $file_format eq '0')) {
@@ -972,6 +977,8 @@ sub file_size_info {
 	$file_format = undef;
     }
 
+    $file_format = undef if $file_format && $file_format eq 'auto-detect';
+
     my $st = File::stat::stat($filename);
 
     if (!defined($st)) {