diff --git a/lib/proxy_sha256.py b/lib/proxy_sha256.py
index 605af08..a0353b1 100644
--- a/lib/proxy_sha256.py
+++ b/lib/proxy_sha256.py
@@ -50,12 +50,12 @@ class ProxySHA256:
         sha = hashlib.sha256()
         while True:
             buffer = fin.read(10000)
-            if not buffer:
-                break
-            sha.update(buffer)
             # only guess from the first 10K
             if not mimetype:
                 mimetype = self.mime.from_buffer(buffer)
+            if not buffer:
+                break
+            sha.update(buffer)
         fin.close()
         LFSOid(self.db).add(
             project, package, name, revision, sha.hexdigest(), size, mimetype, file_md5