diff --git a/src/backend/core/migrations/00013_fix_content_types.py b/src/backend/core/migrations/00013_fix_content_types.py
new file mode 100644
index 000000000..0700c255f
--- /dev/null
+++ b/src/backend/core/migrations/00013_fix_content_types.py
@@ -0,0 +1,100 @@
+import magic
+from django.db import migrations
+from django.core.files.storage import default_storage
+
+def fix_attachments_content_type(apps, schema_editor):
+    """
+    Iterate over all Document objects and fix ContentType for files in
+    <doc_id>/attachments/ prefix on default_storage (MinIO).
+    """
+
+    Document = apps.get_model('core', 'Document')
+
+    s3_client = default_storage.connection.meta.client
+    bucket_name = default_storage.bucket_name
+
+    mime_detector = magic.Magic(mime=True)
+
+    documents = Document.objects.all()
+    print(f"[INFO] Found {documents.count()} documents. Starting ContentType fix...")
+
+    for doc in documents:
+        doc_id_str = str(doc.id)
+        prefix = f"{doc_id_str}/attachments/"
+        print(f"[INFO] Processing attachments under prefix '{prefix}' ...")
+
+        continuation_token = None
+        total_updated = 0
+
+        while True:
+            list_kwargs = {
+                "Bucket": bucket_name,
+                "Prefix": prefix
+            }
+            if continuation_token:
+                list_kwargs["ContinuationToken"] = continuation_token
+
+            response = s3_client.list_objects_v2(**list_kwargs)
+
+            # If no objects found under this prefix, break out of the loop
+            if "Contents" not in response:
+                break
+
+            for obj in response["Contents"]:
+                key = obj["Key"]
+
+                # Skip if it's a folder
+                if key.endswith("/"):
+                    continue
+
+                try:
+                    # Get existing metadata
+                    head_resp = s3_client.head_object(Bucket=bucket_name, Key=key)
+                    old_metadata = head_resp.get("Metadata", {})
+
+                    # Read first ~1KB for MIME detection
+                    partial_obj = s3_client.get_object(
+                        Bucket=bucket_name,
+                        Key=key,
+                        Range="bytes=0-1023"
+                    )
+                    partial_data = partial_obj["Body"].read()
+
+                    # Detect MIME type
+                    magic_mime_type = mime_detector.from_buffer(partial_data)
+
+                    # Update ContentType
+                    s3_client.copy_object(
+                        Bucket=bucket_name,
+                        CopySource={"Bucket": bucket_name, "Key": key},
+                        Key=key,
+                        ContentType=magic_mime_type,
+                        Metadata=old_metadata,
+                        MetadataDirective="REPLACE"
+                    )
+                    total_updated += 1
+
+                except Exception as exc:
+                    print(f"[ERROR] Could not update ContentType for {key}: {exc}")
+
+            if response.get("IsTruncated"):
+                continuation_token = response.get("NextContinuationToken")
+            else:
+                break
+
+        if(total_updated > 0):
+          print(f"[INFO] -> Updated {total_updated} objects for Document {doc_id_str}.")
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0012_make_document_creator_and_invitation_issuer_optional'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            fix_attachments_content_type,
+            reverse_code=migrations.RunPython.noop
+        ),
+    ]
\ No newline at end of file
diff --git a/src/backend/core/tests/migrations/test_00013_fix_content_types.py b/src/backend/core/tests/migrations/test_00013_fix_content_types.py
new file mode 100644
index 000000000..12cd297f3
--- /dev/null
+++ b/src/backend/core/tests/migrations/test_00013_fix_content_types.py
@@ -0,0 +1,40 @@
+import uuid
+import pytest
+from core import factories
+from django.core.files.storage import default_storage
+
+@pytest.mark.django_db
+def test_fix_attachments_content_type_migration(migrator):
+    """
+    Test that the migration fixes the ContentType of attachment in our storage.
+    """
+    migrator.apply_initial_migration(('core', '0012_make_document_creator_and_invitation_issuer_optional'))
+
+    doc_id = uuid.uuid4()
+    factories.DocumentFactory(id=doc_id)
+
+    # Put a file with a *wrong* ContentType
+    s3_client = default_storage.connection.meta.client
+    bucket_name = default_storage.bucket_name
+    key = f"{doc_id}/attachments/testfile.png"
+    fake_png = (
+      b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR..."
+    )
+    s3_client.put_object(
+        Bucket=bucket_name,
+        Key=key,
+        Body=fake_png,
+        ContentType="text/plain",
+        Metadata={"owner": "None"}
+    )
+
+    # Apply the migration that fixes the ContentType
+    migrator.apply_tested_migration(('core', '00013_fix_content_types'))
+
+    head_resp = s3_client.head_object(Bucket=bucket_name, Key=key)
+    assert head_resp["ContentType"] == "image/png", (
+        f"ContentType not fixed, got {head_resp['ContentType']!r}"
+    )
+
+    # Check that original metadata was preserved
+    assert head_resp["Metadata"].get("owner") == "None"
\ No newline at end of file