diff --git a/internal/appherder/appimage.go b/internal/appherder/appimage.go
index e33235c..f331c1e 100644
--- a/internal/appherder/appimage.go
+++ b/internal/appherder/appimage.go
@@ -125,10 +125,13 @@ func fileSystemOffset(file io.ReaderAt) (int64, error) {
 	}
 }
 
-// isSquashFS reports whether a SquashFS superblock begins at offset.
-func isSquashFS(file interface {
+// readerAt is the subset of io.ReaderAt used by filesystem detection helpers.
+type readerAt interface {
 	ReadAt([]byte, int64) (int, error)
-}, offset int64) bool {
+}
+
+// isSquashFS reports whether a SquashFS superblock begins at offset.
+func isSquashFS(file readerAt, offset int64) bool {
 	magic := make([]byte, 4)
 	_, err := file.ReadAt(magic, offset)
 	return err == nil && string(magic) == "hsqs"
@@ -136,9 +139,7 @@ func isSquashFS(file interface {
 
 // scanForSquashFS searches for a SquashFS superblock in 4096-byte steps over
 // the next 64 MiB starting from offset. Returns the found position or false.
-func scanForSquashFS(file interface {
-	ReadAt([]byte, int64) (int, error)
-}, offset int64) (int64, bool) {
+func scanForSquashFS(file readerAt, offset int64) (int64, bool) {
 	const window = 64 * 1024 * 1024
 	buf := make([]byte, 4096)
 	for pos := offset; pos < offset+window; pos += 4096 {
diff --git a/internal/appherder/dwarfs.go b/internal/appherder/dwarfs.go
index 548c303..c2dcfcb 100644
--- a/internal/appherder/dwarfs.go
+++ b/internal/appherder/dwarfs.go
@@ -56,9 +56,7 @@ func extractCommand(ctx context.Context, appimagePath, destDir string) *exec.Cmd
 }
 
 // isDwarFS reports whether the payload at offset starts with the DwarFS magic.
-func isDwarFS(file interface {
-	ReadAt([]byte, int64) (int, error)
-}, offset int64) bool {
+func isDwarFS(file readerAt, offset int64) bool {
 	magic := make([]byte, 6)
 	_, err := file.ReadAt(magic, offset)
 	return err == nil && string(magic) == "DWARFS"