diff --git a/apps/server/src/routes/api/files.ts b/apps/server/src/routes/api/files.ts
index 4a6e17382b..078feed170 100644
--- a/apps/server/src/routes/api/files.ts
+++ b/apps/server/src/routes/api/files.ts
@@ -232,6 +232,10 @@ function uploadModifiedFileToAttachment(req: Request) {
     const { attachmentId } = req.params;
     const { filePath } = req.body;
 
+    if (!createdTemporaryFiles.has(filePath)) {
+        throw new ValidationError(`File '${filePath}' is not a temporary file.`);
+    }
+
     const attachment = becca.getAttachmentOrThrow(attachmentId);
 
     log.info(`Updating attachment '${attachmentId}' with content from '${filePath}'`);