Offline MP4 video fails with "Sorry" error — incorrectly opened via PreviewImageActivity instead of video player

[676f6f616363]

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• This is a bug, not a question or an enhancement.
• I've searched for similar issues and didn't find a duplicate.
• I've written a clear and descriptive title for this issue, not just "Bug" or "Crash".
• I agree to follow Nextcloud's Code of Conduct.

Steps to reproduce

1. Upload an MP4 video file to Nextcloud (e.g. in a folder that also contains photos).
2. In the Android app, open the file and confirm it plays correctly via streaming (file not marked as offline).
3. In the file's options menu, enable "Available offline".
4. Wait for the download/sync to fully complete (offline icon appears on the file).
5. Tap the file to play it.

Expected behaviour

The video should play normally using the app's video player, the same as it does when streamed.

Actual behaviour

The video player fails to open. Instead, the app shows a "Sorry" error screen. Disabling "Available offline" (removing the local copy) and playing the same file via streaming works without issue. Re-enabling "Available offline" reproduces the failure consistently.

adb logcat captured during the failure shows the app launching com.owncloud.android.ui.preview.PreviewImageActivity (the image preview screen) for the offline MP4 file, instead of the video player activity:

E PreviewImageFragment: File could not be loaded as a bitmap: /storage/emulated/0/Android/media/com.nextcloud.client/nextcloud/[account]/[folder path]/20260802_164950.mp4

The file is being routed to the image-preview component, which fails to decode it as a bitmap since it's a video — producing the "Sorry" error. When the same file is streamed directly from the server (not offline), it is correctly opened via the video player (AndroidXMedia3/ExoPlayer), confirmed by log lines such as:

I ExoPlayerImpl: Init [...] [AndroidXMedia3/1.10.1] [...]

This suggests the local (offline-cached) copy of the file is not being correctly identified as a video by MIME type / file-type detection, and the app falls back to (or incorrectly selects) the image preview pathway instead of the video player, only for locally cached files.

Android version

16

Device brand and model

Samsung Galaxy S24 Ultra

Stock or custom OS?

Stock

Nextcloud android app version

34.1.0

Nextcloud server version

34.0.2

Using a reverse proxy?

Yes

Android logs

--- Sanitized adb logcat excerpt (account/domain and personal file path redacted) ---

08-05 18:56:07.344 12983 12983 D FileDisplayActivity: file download completed received
08-05 18:56:07.347 12983 12983 D FileDisplayActivity: updating ui for file download

[user taps the downloaded MP4 file]

08-05 18:56:08.610 2998 7323 D ActivityTaskManager: startActivityAsUser: callingPid=12983, callingUid=10061
08-05 18:56:08.621 2998 7323 I ActivityTaskManager: START u0 {xflg=0x4 cmp=com.nextcloud.client/com.owncloud.android.ui.preview.PreviewImageActivity (has extras)} with LAUNCH_MULTIPLE from uid 10061 (com.nextcloud.client) result code=0
08-05 18:56:08.637 12983 12983 D PreviewImageActivity: onCreate(Bundle) starting
08-05 18:56:08.660 12983 12983 D PreviewImageActivity: onStart() starting
08-05 18:56:08.666 12983 12983 D PreviewImageActivity: onResume() starting
08-05 18:56:08.687 12983 12983 D PreviewImageFragment: savedInstanceState is null
08-05 18:56:08.694 12983 7060 E PreviewImageFragment: File could not be loaded as a bitmap: /storage/emulated/0/Android/media/com.nextcloud.client/nextcloud/[account]@[domain]/[folder path]/20260802_164950.mp4
08-05 18:56:08.702 2998 3185 I ActivityTaskManager: Displayed com.nextcloud.client/com.owncloud.android.ui.preview.PreviewImageActivity for user 0: +88ms

--- End of excerpt ---

Server error logs

Additional information

Key observation: Immediately after the offline download completes (FileDisplayActivity: "file download completed received"),
tapping the file directly launches PreviewImageActivity (the image preview screen) rather than any video player activity.
No video/MIME-type detection or routing to a video player activity is visible in the log at any point in this sequence.
The activity attempts to decode the MP4 as a bitmap and fails, producing the "Sorry" error shown to the user.

For comparison, streaming the same file (not offline) correctly initializes ExoPlayer (AndroidXMedia3) instead of
PreviewImageActivity.