diff --git a/src/workflow/diff/ArcanistDiffWorkflow.php b/src/workflow/diff/ArcanistDiffWorkflow.php
index 3e602793..3db0f813 100644
--- a/src/workflow/diff/ArcanistDiffWorkflow.php
+++ b/src/workflow/diff/ArcanistDiffWorkflow.php
@@ -898,11 +898,9 @@ EOTEXT
       return $result;
     }
 
-    $future = new ExecFuture('file -b --mime -');
-    $future->write($data);
-    list($mime_type) = $future->resolvex();
-
-    $mime_type = trim($mime_type);
+    $tmp = new TempFile();
+    Filesystem::writeFile($tmp, $data);
+    $mime_type = Filesystem::getMimeType($tmp);
     $result['mime'] = $mime_type;
 
     echo "Uploading {$desc} '{$name}' ({$mime_type}, {$size} bytes)...\n";
diff --git a/src/workflow/diff/__init__.php b/src/workflow/diff/__init__.php
index db89e37a..ec97b758 100644
--- a/src/workflow/diff/__init__.php
+++ b/src/workflow/diff/__init__.php
@@ -23,6 +23,7 @@ phutil_require_module('phutil', 'console');
 phutil_require_module('phutil', 'console/editor');
 phutil_require_module('phutil', 'filesystem');
 phutil_require_module('phutil', 'filesystem/filelist');
+phutil_require_module('phutil', 'filesystem/tempfile');
 phutil_require_module('phutil', 'future/exec');
 phutil_require_module('phutil', 'utils');