I have the same problem with an Id generated by Mongodb plugin ( ObjectId ).
My workaround solution
define a page context attribute with g:set tag to build a single quote string like value="'${instance.id}'"
use that attribute as value for my formData map entry
Copy the plugin Tag inside my project and change the code that constructs requests by deleting the single quotes
around map enty values ( extra ones due to my attribue format )
if using the formData attribute with string values the upload does not work anymore, because you get syntax errors in the generated javascript code.
e.g. formData="[p1:new Date()]"
Fix is to quote the values in line 214 in BootstrapFileUploadTagLib.groovy: