I've found attaching an XML file as an attachment via sp_send_dbmail very troublesome.
I did this with WhoIsActive which is probably a little more complicated than you need.http://jonmorisissqlblog.blogspot.com/
Basically the way I figured it out was to create a stored procedure. Within the stored proc I run a select FOR xml path, but return it as NVARCHAR(max). Then I called this stored proc within the @query option of sp_send_dbmail.
@profile_name = '[Database Mail Profile]',
@subject = 'Who Is Active',
@body_format = 'TEXT',
@query = 'SET NOCOUNT ON; SET ANSI_WARNINGS OFF; DECLARE @bodyXMLreturn nvarchar(max) EXEC dbo.sp_WhoIsActiveXMLout @bodyXMLreturn OUTPUT select @bodyXMLreturn',
@execute_query_database = '[Database]',
@attach_query_result_as_file = 1,
@query_attachment_filename = 'WhoIsActive.xml',
@query_result_header = 0,
@query_result_width = 32767,
@query_result_separator = '',
@exclude_query_output = 0,
@query_result_no_padding = 0,