Be lenient as we can reasonably assume this is a ZIP file

Author: vmcj

webapp/src/Service/ExternalContestSourceService.php

@@ -1438,13 +1438,18 @@ protected function importSubmission(Event $event, EventData $data): void
                     'message' => 'No source files in event',
                 ]);
                 $submissionDownloadSucceeded = false;
-            } elseif (($data->files[0]->mime ?? null) !== 'application/zip') {
+            } elseif ($data->files[0]->mime !== null && $data->files[0]->mime !== 'application/zip') {
                 $this->addOrUpdateWarning($event, $data->id, ExternalSourceWarning::TYPE_SUBMISSION_ERROR, [
                     'message' => 'Non-ZIP source files in event',
                 ]);
                 $submissionDownloadSucceeded = false;
             } else {
                 $zipUrl = $data->files[0]->href;
+                if (!isset($data['files'][0]['mime'])) {
+                    $this->addOrUpdateWarning($event, $data->id, ExternalSourceWarning::TYPE_SUBMISSION_ERROR, [
+                        'message' => 'MIME type not set, should be "application/zip"',
+                    ]);
+                }
                 if (preg_match('/^https?:\/\//', $zipUrl) === 0) {
                     // Relative URL, prepend the base URL.
                     $zipUrl = ($this->basePath ?? '') . $zipUrl;