set myPrompt to "Where is the file 'info.xml?'"
tell application "Finder" to set myTest to (choose file with prompt myPrompt) as text
seems to do the job and would be easy if the file remains unchanged, but unpractical for arbitrary selections.
4. In that case, I would suggest something along the following lines:
set myTest to "info.xml" & return
tell application "Finder" to set myList to the selection as text
set the text item delimiters to return
if myTest is not in (myList as string) & return then
display dialog myTest & " is missing!"
else
# do your stuff
end if