Why not support normal BMS tags?

The formats usually used in BMS is not suitable for web consumption.

Eyecatch images #STAGEFILE and background images #BACKBMP are usually BMP files.
BGA are usually WMV, MPG, and AVI files. These file types are proprietary and not widely supported on web browsers.

Bemuse’s limitations

These images, movies and previews are per-song. All BMS files will use the same image.

For this example, I am going to use LeaF’s song about flower.

At first, the server looks like this:

Add a file named eyecatch_image.png.

Bemuse will now display an eyecatch image while loading song.

Add a file named back_image.png.

Bemuse will now display an background image when playing.

Add a BGA file:

Create a text file called README.md.

And put in:

---
video_file: BGA.mp4
video_offset: 1.268
---

Save the file and run bemuse-tools index again.

Refresh the game (you may have to clear the cache). BGA should work now!

Note: I recommend using the WebM format with size 1280x720 and bitrate of 2000kbps.

Convert the BMS into a WAV file (don’t forget to normalize to prevent clipping).
Use Audacity with ReplayGain Plugin.
Open the WAV file in Audacity.
Effect → ReplayGain → Normalize with adjusted output of 4 dB.
Cut the track to 30 seconds.
Apply Fade in at to the first 0.5 seconds.
Apply Fade out at the last 3 seconds.
Export the file to _bemuse_preview.mp3 with bitrate of 128kbps.

Now, you should hear the music preview!