diff --git a/src/ContentMessages.ts b/src/ContentMessages.ts
index 5ad1c61e941..9a6d4da1006 100644
--- a/src/ContentMessages.ts
+++ b/src/ContentMessages.ts
@@ -595,12 +595,6 @@ export default class ContentMessages {
         // if we have a mime type for the file, add it to the message metadata
         if (file.type) {
             content.info.mimetype = file.type;
-
-            // clobber quicktime muxed files to be considered MP4 so browsers
-            // are willing to play them
-            if (content.info.mimetype == "video/quicktime") {
-                content.info.mimetype = "video/mp4";
-            }
         }
 
         const prom = new Promise<void>((resolve) => {
diff --git a/src/components/views/messages/MVideoBody.tsx b/src/components/views/messages/MVideoBody.tsx
index 1b6bafd7d93..96516138a09 100644
--- a/src/components/views/messages/MVideoBody.tsx
+++ b/src/components/views/messages/MVideoBody.tsx
@@ -190,12 +190,21 @@ export default class MVideoBody extends React.PureComponent<IBodyProps, IState>
                 } else {
                     logger.log("NOT preloading video");
                     const content = this.props.mxEvent.getContent<IMediaEventContent>();
+
+                    let mimetype = content?.info?.mimetype;
+
+                    // clobber quicktime muxed files to be considered MP4 so browsers
+                    // are willing to play them
+                    if (mimetype == "video/quicktime") {
+                        mimetype = "video/mp4";
+                    }
+
                     this.setState({
                         // For Chrome and Electron, we need to set some non-empty `src` to
                         // enable the play button. Firefox does not seem to care either
                         // way, so it's fine to do for all browsers.
-                        decryptedUrl: `data:${content?.info?.mimetype},`,
-                        decryptedThumbnailUrl: thumbnailUrl || `data:${content?.info?.mimetype},`,
+                        decryptedUrl: `data:${mimetype},`,
+                        decryptedThumbnailUrl: thumbnailUrl || `data:${mimetype},`,
                         decryptedBlob: null,
                     });
                 }