diff --git a/src/dc_tools.c b/src/dc_tools.c
index 87789bf5..2181bf8c 100644
--- a/src/dc_tools.c
+++ b/src/dc_tools.c
@@ -1429,6 +1429,21 @@ void dc_make_rel_path(dc_context_t* context, char** path)
 }
 
 
+/**
+ * Check if a path describes a file in the blob directory.
+ * The path can be give absolute or relative (starting with `$BLOBDIR`).
+ * The function does not check if the file really exists.
+ */
+int dc_is_in_blobdir(dc_context_t* context, const char* path)
+{
+	if ((strncmp(path, context->blobdir, strlen(context->blobdir))==0)
+	 || (strncmp(path, "$BLOBDIR", 8)==0)) {
+		return 1;
+	}
+	return 0;
+}
+
+
 /**
  * Copy a file to the blob directory, if needed.
  *
@@ -1447,8 +1462,7 @@ int dc_make_rel_and_copy(dc_context_t* context, char** path)
 		goto cleanup;
 	}
 
-	if ((strncmp(*path, context->blobdir, strlen(context->blobdir))==0)
-	 || (strncmp(*path, "$BLOBDIR", 8)==0)) {
+	if (dc_is_in_blobdir(context, *path)) {
 		dc_make_rel_path(context, path);
 		success = 1; // file is already in blobdir
 		goto cleanup;
diff --git a/src/dc_tools.h b/src/dc_tools.h
index 844224c2..5d7aa6e2 100644
--- a/src/dc_tools.h
+++ b/src/dc_tools.h
@@ -110,6 +110,7 @@ int      dc_create_folder           (dc_context_t*, const char* pathNfilename);
 int      dc_write_file              (dc_context_t*, const char* pathNfilename, const void* buf, size_t buf_bytes);
 int      dc_read_file               (dc_context_t*, const char* pathNfilename, void** buf, size_t* buf_bytes);
 char*    dc_get_fine_pathNfilename  (dc_context_t*, const char* pathNfolder, const char* desired_name);
+int      dc_is_in_blobdir           (dc_context_t*, const char* path);
 void     dc_make_rel_path           (dc_context_t*, char** pathNfilename);
 int      dc_make_rel_and_copy       (dc_context_t*, char** pathNfilename);