force flush on temp file during consumption

file.write() does not guarantee that a file handle contains anything
without calling an accompanying file.flush()
For typical files that are larger than the OS-file buffer, this is no
problem
For small files (e.g. 64 characters in a .TXT), this race condition
leads to `inode/x-empty` because the file inode is created, but no
content is written to it

src/documents/views.py

@@ -383,6 +383,7 @@ class PostDocumentView(APIView):
                                          dir=settings.SCRATCH_DIR,
                                          delete=False) as f:
             f.write(doc_data)
+            f.flush()
             os.utime(f.name, times=(t, t))
 
             async_task("documents.tasks.consume_file",