image file URL generated by image_tag include some parameter

I'm using image_tag to generate img tag.

URL of image file generated by image_tag include a parameter like following.

"http:// ..... /images/filename.gif?123123123"

What's this number parameter for? How can I remove it? I'm using rails
with built-in WEBrick server in test mode.

It's a timestamp that is automatically generated by the image_tag()
helper. This forces browsers to download the latest image if updated.

Can I remove it?

Some clients with old browser complain browser can't display images
htmlized with image_tag method. (But I didn't see the situation in
person, and not sure about a cause)

Wow - that must be a pretty old browser...

Just use an img html tag or override the image_tag() helper.

the point is ... Environment varible RAILS_ASSET_ID changes behavior
of rails URL processing.