diff --git a/refinery/lib/structures.py b/refinery/lib/structures.py
index 297e0db5d..dc5ca6c3f 100644
--- a/refinery/lib/structures.py
+++ b/refinery/lib/structures.py
@@ -65,7 +65,7 @@ def __exit__(self, *args):
         self.stream.seek(self.cursor, io.SEEK_SET)
 
 
-class MemoryFile(Generic[T], io.IOBase):
+class MemoryFile(Generic[T]):
     """
     A thin wrapper around (potentially mutable) byte sequences which gives it the
     features of a file-like object.
diff --git a/refinery/lib/vfs.py b/refinery/lib/vfs.py
index 5f73bcb2a..33d7d2ef2 100644
--- a/refinery/lib/vfs.py
+++ b/refinery/lib/vfs.py
@@ -54,10 +54,18 @@ def mmap(self, length: int = 0, offset: int = 0) -> MemoryFile:
         Emulate the result of an `mmap` call to the virtual file.
         """
         view = memoryview(self.data)
+        node = self.node
         if length:
             view = view[offset:offset + length]
-        fd = MemoryFile(view, read_as_bytes=True, fileno=self.node)
-        return fd
+
+        class _MappedView(bytearray, MemoryFile):
+            def __init__(self):
+                MemoryFile.__init__(self, self, True, node)
+
+        mapped = _MappedView()
+        mapped[:] = view
+
+        return mapped
 
     def open(self, mode: str) -> MemoryFile:
         """