image_tag returns screwy src url from url_for_file_column

Seems like I had this question, but never asked or never resolved it.
Anyway,
I'm using file_column. I've been using it.
No problems. Works great.
url_for_file_column returns the proper url every time.
Except... when I call url_for_file_column from within image_tag it
returns an img element where the source string seems to contain some
extra string of numbers that looks like a Ruby object id.

I'm getting:

<img alt="Ebony Snare" src="/item/image/5/thumbnail/
ebony_snare_shell_a.jpg?1186961518" />

should be:
<img alt="Ebony Snare" src="/item/image/5/thumbnail/
ebony_snare_shell_a.jpg" />

My show view has this:

<%= image_tag url_for_file_column(@item, "image", "thumbnail"), :alt
=> @item.name %>

I tried it like this with optional parenthesis and nothing was
different:

<%= image_tag( url_for_file_column(@item, "image", "thumbnail"), :alt
=> @item.name) %>

There is nothing that seems abnormal about this code to me.

I'm also curious why the alt appears before the src attribute, I know
that it doesn't matter, but it doesn't show that way in the docs...

Any suggestions?? (short of assigning the url_for_file_column return
to an intermediary variable)

actually, it turns out, this number string is appearing even with
simply using image_tag and a src image in the public images dir!!??
What's this all about?
Is this the only way to get img elements in Rails? Will they always
return src attributes with strange numbers appended?

(* . *)~

Well, searching online, the best I've come up with is that apparently
Rails uses this to prevent browser caching...? (and I guess hotlinking
to images)

I just wonder if there is a way to turn off this feature?