fix(server): return correct headers

Thanks to https://www.reddit.com/r/nginx/comments/rdm7yo/resume_downloads_send_through_xaccelredirect/ho4b7a4/
for the reference!

server/core/models.py

@@ -1,3 +1,5 @@
+import os
+
 import humanize
 from auditlog.registry import auditlog
 from constance import config
@@ -396,6 +398,9 @@ def human_readable_timedelta(self):
             humanize.i18n.deactivate()
         return humanize.naturaltime(date.today() - self.build_date)
 
+    def zip_file_basename(self):
+        return os.path.basename(self.zip_file.name)
+
 
 # Statistics model
 class Statistics(models.Model):

server/downloads/views.py

@@ -1,3 +1,6 @@
+import os
+import time
+
 from django.http import HttpResponse
 from django.shortcuts import get_object_or_404
 from django.urls import reverse
@@ -63,7 +66,13 @@ def download_check_view(request, codename, file_name):
 
     response = HttpResponse()
     response["X-Accel-Redirect"] = f"/internal/media/{build.zip_file.name}"
-    response["Content-Type"] = ""
+    response["Last-Modified"] = time.strftime(
+        "%a,%e %b %Y %H:%M:%S %Z", time.gmtime(os.path.getmtime(build.zip_file.name))
+    )
+    response[
+        "Content-Disposition"
+    ] = f'attachment; filename="{build.zip_file_basename()}"'
+    response["Content-Type"] = "application/octet-stream"
 
     if build.is_archived:
         limit_speed = config.SHIPPER_DOWNLOADS_ARCHIVE_THROTTLE * 1000 * 1000