On Discord's Android app, it's possible to send an image with spoiler by renaming this image in such a way it's name starts with "SPOILER_". Spoilers4Discord is now available on your Android device (5.0 and above) to help you by renaming an image to send automaticaly.
How it work:
Spoilers4Discord will ask you for an image to rename, then this image will be copied into the app's cache with the correct name (so the original image doesn't change name). Then Discord will ask you where to send the renamed image.
The content of the cache is emptied each time you're promted to choose an image.