[hotfix] multipart body contents in XQuery

This reverts commit c22ffe0.

It turns out that payloads in body encoded as  multipart/form-data
have to be written to disk in order to be available in XQuery context.
This is the reason the DiskFileItemFactory threshhold has to be zero.

Author: line-o

exist-core/src/main/java/org/exist/http/servlets/HttpRequestWrapper.java

@@ -188,6 +188,9 @@ private void parseMultipartContent() {
         // Create a factory for disk-based file items
         final DiskFileItemFactory factory = new DiskFileItemFactory();
 
+        // Ensure small attachments are readable in XQuery context
+        factory.setSizeThreshold(0);
+
         // Create a new file upload handler
         final ServletFileUpload upload = new ServletFileUpload(factory);