Copy attachments from one feature layer to another

Question

I'm using the MergeLayers task to combine two point feature layers together. I got it working fine except it did not copy the attachments that both feature layers contain. This is described here, but I used the REST API, not ArcGIS Online.
Is there any way to copy/paste attachments from one feature layer to another (possibly with a match on the GlobalID) ?

I can always make a copy of the first layer, export the second as a FGDB and then append this to the copy, but it involves more step, so I wonder if there is a simpler solution to manage & move attachments between layers.

Thanks!

mikaël · Accepted Answer

Well I managed to copy attachments from one feature service to another as I posted here, although I have to download them as string, decode and upload again. A server side solution would be best but this works.