diff --git a/lib/Providers/Personal/PersonalService.php b/lib/Providers/Personal/PersonalService.php
index f1146be..b1e9214 100644
--- a/lib/Providers/Personal/PersonalService.php
+++ b/lib/Providers/Personal/PersonalService.php
@@ -712,8 +712,8 @@ class PersonalService implements ServiceBaseInterface, ServiceCollectionMutableI
 			throw new InvalidParameterException("Entity not found: $identifier");
 		}
 
-		// detect MIME type and format from content header
-		$signature = Signature::detect(substr($data, 0, Signature::HEADER_SIZE));
+		// detect MIME type and format from content (full content lets us tell OOXML/ODF apart from a plain zip)
+		$signature = Signature::detect(substr($data, 0, Signature::SAMPLE_SIZE), $data);
 
 		// Write blob data
 		$size = $this->blobStore->blobWrite((string)$identifier, $data);
@@ -779,7 +779,7 @@ class PersonalService implements ServiceBaseInterface, ServiceCollectionMutableI
 		// Detect MIME type and format from first chunk (offset === 0)
 		$signature = null;
 		if ($offset === 0) {
-			$signature = Signature::detect(substr($data, 0, Signature::HEADER_SIZE));
+			$signature = Signature::detect(substr($data, 0, Signature::SAMPLE_SIZE), $data);
 		}
 
 		$bytes = $this->blobStore->blobWriteChunk((string)$identifier, $offset, $data);