diff --git a/mongoose.c b/mongoose.c
index 4f28969d..0c1df381 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -691,7 +691,7 @@ int mg_http_upload(struct mg_connection *c, struct mg_http_message *hm,
     snprintf(path, sizeof(path), "%s%c%s", dir, MG_DIRSEP, name);
     LOG(LL_DEBUG,
         ("%p %d bytes @ %d [%s]", c->fd, (int) hm->body.len, (int) oft, name));
-    if ((fp = fopen(path, oft == 0 ? "wb" : "a")) == NULL) {
+    if ((fp = fopen(path, oft == 0 ? "wb" : "ab")) == NULL) {
       mg_http_reply(c, 400, "", "fopen(%s): %d", name, errno);
       return -2;
     } else {
diff --git a/src/http.c b/src/http.c
index b0a7258a..9d81fbc6 100644
--- a/src/http.c
+++ b/src/http.c
@@ -271,7 +271,7 @@ int mg_http_upload(struct mg_connection *c, struct mg_http_message *hm,
     snprintf(path, sizeof(path), "%s%c%s", dir, MG_DIRSEP, name);
     LOG(LL_DEBUG,
         ("%p %d bytes @ %d [%s]", c->fd, (int) hm->body.len, (int) oft, name));
-    if ((fp = fopen(path, oft == 0 ? "wb" : "a")) == NULL) {
+    if ((fp = fopen(path, oft == 0 ? "wb" : "ab")) == NULL) {
       mg_http_reply(c, 400, "", "fopen(%s): %d", name, errno);
       return -2;
     } else {
diff --git a/test/unit_test.c b/test/unit_test.c
index 0a233d9e..c069a830 100644
--- a/test/unit_test.c
+++ b/test/unit_test.c
@@ -574,8 +574,12 @@ static void test_http_server(void) {
                  "POST /upload?name=uploaded.txt HTTP/1.0\r\n"
                  "Content-Length: 5\r\n"
                  "\r\nhello") == 200);
+    ASSERT(fetch(&mgr, buf, url,
+                 "POST /upload?name=uploaded.txt&offset=5 HTTP/1.0\r\n"
+                 "Content-Length: 6\r\n"
+                 "\r\n\nworld") == 200);
     ASSERT((p = mg_file_read("uploaded.txt")) != NULL);
-    ASSERT(strcmp(p, "hello") == 0);
+    ASSERT(strcmp(p, "hello\nworld") == 0);
     free(p);
     remove("uploaded.txt");
   }