diff --git a/lib/file/getFileBlobAsync.js b/lib/file/getFileBlobAsync.js
index 4c845d9..45c6c19 100644
--- a/lib/file/getFileBlobAsync.js
+++ b/lib/file/getFileBlobAsync.js
@@ -2,10 +2,23 @@ import { extname } from 'path';
 
 import { ENCODING, FILE_EXTENSION } from './constants';
 
+const wrapBlob = blob => {
+  // Cypress version 5 assigns a function with a compatibility warning
+  // to blob.then, but that makes the Blob actually thenable. We have
+  // to remove that to Promise.resolve not treat it as thenable.
+  if (blob instanceof Cypress.Promise) {
+    return blob;
+  }
+
+  // eslint-disable-next-line no-param-reassign
+  delete blob.then;
+  return Cypress.Promise.resolve(blob);
+};
+
 const ENCODING_TO_BLOB_GETTER = {
   [ENCODING.ASCII]: fileContent => Cypress.Promise.resolve(fileContent),
-  [ENCODING.BASE64]: (fileContent, mimeType) => Cypress.Blob.base64StringToBlob(fileContent, mimeType),
-  [ENCODING.BINARY]: (fileContent, mimeType) => Cypress.Blob.binaryStringToBlob(fileContent, mimeType),
+  [ENCODING.BASE64]: (fileContent, mimeType) => wrapBlob(Cypress.Blob.base64StringToBlob(fileContent, mimeType)),
+  [ENCODING.BINARY]: (fileContent, mimeType) => wrapBlob(Cypress.Blob.binaryStringToBlob(fileContent, mimeType)),
   [ENCODING.HEX]: fileContent => Cypress.Promise.resolve(fileContent),
   [ENCODING.LATIN1]: fileContent => Cypress.Promise.resolve(fileContent),
   [ENCODING.UTF8]: fileContent => Cypress.Promise.resolve(fileContent),