I'm trying to write an AppleScript for use with Mail (on Snow Leopard) to save image attachments of messages to a folder. The main part of the AppleScript is:

property ImageExtensionList : {"jpg", "jpeg"}
property PicturesFolder : path to pictures folder as text
property SaveFolderName : "Fetched"
property SaveFolder : PicturesFolder & SaveFolderName

tell application "Mail"
  set theMessages to the selection
  repeat with theMessage in theMessages
    repeat with theAttachment in every mail attachment of theMessage
      set attachmentFileName to theAttachment's name
      if isImageFileName(attachmentFileName) then
        set attachmentPathName to SaveFolder & attachmmentFileName
        save theAttachment in getNonexistantFile(attachmentPathName)
      end if        
    end repeat
  end repeat
end tell

on isImageFileName(theFileName)
  set dot to offset of "." in theFileName
  if dot > 0 then
    set theExtension to text (dot + 1) thru -1 of theFileName
    return theExtension is in ImageExtensionList
  end if
  return false
end isImageFileName

When run, I get the error:

error "Mail got an error: Can’t continue isImageFileName." number -1708

where error -1708 is:

Event wasnt handled by an Apple event handler.

However, if I copy/paste the isImageFileName() into another script like:

property imageExtensionList : {"jpg", "jpeg"}

on isImageFileName(theFileName)
  set dot to offset of "." in theFileName
  if dot > 0 then
    set theExtension to text (dot + 1) thru -1 of theFileName
    return theExtension is in ImageExtensionList
  end if
  return false
end isImageFileName

if isImageFileName("foo.jpg") then
  return true
else
  return false
end if

it works fine. Why does Mail complain about this?