I didn’t have any problems with the choose file command. I set up a test app and connected the following two bits to menu items in the main menu.
on choose menu item theObject
if name of theObject is "chooseFile" then
set thePrompt to "Choose the file you want to open..."
set theTypes to {"TEXT", "APPL"}
set theMultiples to false
set theLocation to "Macintosh HD:Users:jed:Desktop:" as alias
set myFile to choose file of type theTypes with prompt thePrompt default location theLocation multiple selections allowed theMultiples without invisibles
display dialog (myFile as string)
(* I used this to check for wierd file types *)
else if name of theObject is "getFileType" then
set thePrompt to "Choose the file you want the type for..."
set myFile to choose file with prompt thePrompt
set theType to file type of (info for myFile) as string
display dialog theType
end if
end choose menu item