Hi Lauren,
There are 2 ways of building widgets, the old way names custom widgets and the new way pluggable widgets.
The new way is till in development has not reached parity however the API is more powerful. see:
https://docs.mendix.com/apidocs-mxsdk/apidocs/pluggable-widgets#2-differences-between-pluggable-and-custom-widgets So it was not removed… just not jet added
I hope they will add file upload support soon… but did not hear anything on the roadmap.
The magic Jelte used, he combined React with the old API.
Some workaround used for files (if they are not to big) is to convert the file into base64 and set it as a value in in a string attribute of the object. On the server side convert it to a file.
Cheers, Andries