Use Export Field Contents to export a copy of the file to the Temporary folder. You can set a path to the temporary folder for use with Export Field Contents.
Then specify this $Path variable as the attachment in the Send Mail Dialog.
By exporting to the temporary folder, the file will not persist when FileMaker is closed and thus you don't clutter up the local user's hard drive with extra copies of these files.
For more on $Path Variables, container fields and the scripts that use them, see: Exploring the use of a $Path Variable in Scripts