Don't compute MIME type of noninitial chunks from diffusion.filecontentquery

Summary:
Ref T12857. This is generally fairly fuzzy for now, but here's something concrete: when we build a large file with `diffusion.filecontentquery`, we compute the MIME type of all chunks, not just the initial chunk.

Instead, pass a dummy MIME type to non-initial chunks so we don't try to compute them. This mirrors logic elsewhere, in `file.uploadchunk`. This should perhaps be centralized at some point, but it's a bit tricky since the file doesn't know that it's a chunk until later.

Also, clean up the `TempFile` immediately -- this shouldn't actually affect anything, but we don't need it to live any longer than this.

Test Plan:
  - Made `hashFileContent()` return `null` to skip the chunk cache.
  - Added `phlog()` to the MIME type computation.
  - Loaded a 12MB file in Diffusion.
  - Before patch: Saw 3x MIME type computations, one for each 4MB chunk.
  - After patch: Saw 1x MIME type computation, for initial chunk only.

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T12857

Differential Revision: https://secure.phabricator.com/D18138

src/applications/files/storage/PhabricatorFile.php

@@ -393,6 +393,7 @@ final class PhabricatorFile extends PhabricatorFileDAO
       $tmp = new TempFile();
       Filesystem::writeFile($tmp, $data);
       $file->setMimeType(Filesystem::getMimeType($tmp));
+      unset($tmp);
     }
 
     try {

src/applications/files/uploadsource/PhabricatorFileUploadSource.php

@@ -186,12 +186,18 @@ abstract class PhabricatorFileUploadSource
     $actual_length = strlen($data);
     $rope->removeBytesFromHead($actual_length);
 
-    $chunk_data = PhabricatorFile::newFromFileData(
-      $data,
-      array(
+    $params = array(
       'name' => $file->getMonogram().'.chunk-'.$offset,
       'viewPolicy' => PhabricatorPolicies::POLICY_NOONE,
-      ));
+    );
+
+    // If this isn't the initial chunk, provide a dummy MIME type so we do not
+    // try to detect it. See T12857.
+    if ($offset > 0) {
+      $params['mime-type'] = 'application/octet-stream';
+    }
+
+    $chunk_data = PhabricatorFile::newFromFileData($data, $params);
 
     $chunk = PhabricatorFileChunk::initializeNewChunk(
       $file->getStorageHandle(),