Yes, I have double-checked that these macros are not being provided in the released ambd_sdk. You can access these macros only after signing the NDA. A guide for signing NDA from here.
The reason for modifying the maxlength in your HTML code is trying to see whether this attribute limits the upload size of your file. As a reference, the html_input_file code from W3School didn’t seem to specify the max length attribute.