Email requires a physical file for the attachment so you have to export the file to disk.
Use the temporary folder and your OS should eventually delete this file.
You'll need a pathname so use Get(temporaryfolder) and append the filename with prefix to it. Unfortunately FileMaker doesn't use the actual file name but shortens it File rather than File.fmp12.
So, in case the suffix changes create a list out of the get(filepath) using substitute and then the last item is the filename.
I'll leave the final parse of the pathname to you but if you need, just ask and I'll post it.
Set a variable to the filepath and use the variable in the dialog.
You can export from the container to the path, then set the same variable in the Send Mail dialog (not the actual path to the document).
see if these help:
Get(FilePath) is for active filemaker file, there is nothing to do with file that exported from container.
Filename can be get using GetContainerAttribute(container;"filename") or GetAsText(container) on older FM versions.