use the mimetype from the info property rather than the EncryptedFile

the mimetype in EncryptedFile is undocumented and redundant.
see https://github.com/matrix-org/matrix-doc/pull/2582

src/utils/DecryptFile.ts

@@ -17,18 +17,22 @@ limitations under the License.
 // Pull in the encryption lib so that we can decrypt attachments.
 import encrypt from 'browser-encrypt-attachment';
 import { mediaFromContent } from "../customisations/Media";
-import { IEncryptedFile } from "../customisations/models/IMediaEventContent";
+import { IEncryptedFile, IMediaEventInfo } from "../customisations/models/IMediaEventContent";
 import { getBlobSafeMimeType } from "./blobs";
 
 /**
  * Decrypt a file attached to a matrix event.
- * @param {IEncryptedFile} file The json taken from the matrix event.
+ * @param {IEncryptedFile} file The encrypted file information taken from the matrix event.
  *   This passed to [link]{@link https://github.com/matrix-org/browser-encrypt-attachments}
  *   as the encryption info object, so will also have the those keys in addition to
  *   the keys below.
+ * @param {IMediaEventInfo} info The info parameter taken from the matrix event.
  * @returns {Promise<Blob>} Resolves to a Blob of the file.
  */
-export function decryptFile(file: IEncryptedFile): Promise<Blob> {
+export function decryptFile(
+    file: IEncryptedFile,
+    info: IMediaEventInfo | undefined,
+): Promise<Blob> {
     const media = mediaFromContent({ file });
     // Download the encrypted file as an array buffer.
     return media.downloadSource().then((response) => {
@@ -44,7 +48,7 @@ export function decryptFile(file: IEncryptedFile): Promise<Blob> {
         // they introduce XSS attacks if the Blob URI is viewed directly in the
         // browser (e.g. by copying the URI into a new tab or window.)
         // See warning at top of file.
-        let mimetype = file.mimetype ? file.mimetype.split(";")[0].trim() : '';
+        let mimetype = info?.mimetype ? info.mimetype.split(";")[0].trim() : '';
         mimetype = getBlobSafeMimeType(mimetype);
 
         return new Blob([dataArray], { type: mimetype });