Many tags, such as CFFILE and CFDIRECTORY require you to provide a PHYSICAL path to the target directory or file. To do this, you need to determine your physical directory, and you can do so using the following code:
<cfscript>
webroot = getDirectoryFromPath(expandPath("/index.cfm"));
</cfscript>
The above code will return something similar to the following:
d:\domains\yourdomain\wwwroot\
Note that it includes the trailing backslash, which is important.
Then in CFFILE, if you wanted to put a file, let say in your "uploads" directory, you would use the variable as a prefix for you filename like:
file="#webroot#uploads/thisfile.jpg"