If you run xattr on your downloaded file, like this:
xattr /Path/To/File
You'll get a key named com.apple.metadata:kMDItemWhereFroms. To delete this key in the Terminal, run:
xattr -d com.apple.metadata:kMDItemWhereFroms /Path/To/File
I knew I would have to do this often, so instead of running a shell script every time I wanted to strip the Where From, I wrote an AppleScript:
on deleteWhereFrom(fileToClean) try set posixPath to quoted form of POSIX path of fileToClean do shell script "xattr -d com.apple.metadata:kMDItemWhereFroms " & posixPath end try end deleteWhereFrom on open imgs repeat with i in imgs deleteWhereFrom(i) end repeat end open on run set imgs to choose file with multiple selections allowed repeat with i in imgs deleteWhereFrom(i) end repeat end run
[crarko adds: I tested this, and it works as described.]

