Through tradeinterop you can send an unlimited number of attachments, up to a maximum of 5mb per attachment (with a total of 10mb).
Attachments can easily be sent by email to your customer's email address on the tradeinterop platform. In addition to the .xml file (and possibly the .pdf file), your email will also contain one or more attachments.
Note: the PDF invoice must have the same file name before the dot as the XML invoice. The other attachments may have arbitrary file names. For example:
170413-1234567890.xml (the e-invoice)
170413-1234567890.pdf (the PDF version of the e-invoice)
timespecification.csv (additional appendix)