Out Of the Box in Sitecore 9.0 update 2 there is no upload field for Sitecore Forms? Are you Using the old WFFM (Web Forms For Marketeers) module?
For Sitecore Forms.
1) In Sitecore 9.3 there is a File upload for Sitecore Forms, with size and type check.
2) For Sitecore 9 and Forms you can add this functionality, with Sitecore-Forms-Extensions code here github.com/.../Sitecore-Forms-Extensions