You can actually split the files across multiple textareas to get more space (Base64 is easily splittable and joinable). But that can get unmanageable once you get past 3-4 fields, as you can imagine. I've since found a third-party upload/storage provider that really Gets It (tm) about integrating with independent forms (not just Marketo) and I've been recommending them to my clients. PM me and I'll tell you about them if you want.
Amazing feature! Will it be possible to upload files such as pdf or docx some day?
Either one would work if they're small enough. The irony (if you can call it that given the potential size of images in general) is that because of JPEG compression, outputting a compressed image with fixed dimensions is very likely to stay under the limit, whereas unless DOCX/PDF comes from a template with a set # of pages/fields it could get very huge, very quickly.
I've been reading up on the form you created here MktoForms2 :: Uploadcare - I just wanted to confirm what is the field name of the hidden field you created in the JS and what other actions did you do on the marketo database side to populate the URL into the hidden field?
I've tried recreating the the form you created (same HTML and JS just switched over to the form reference to what my own form as well as using my uploadcare public key) I even named my string field RFPURL however it still does not populate a URL on marketo after submission.
Please let me know your thoughts, happy to provide more information if necessary