Hello,
There is an issue in the IDE where it will buffer the contents of the file into memory while performing the upload. In the rstudio.cloud environment this can cause an issue because the memory footprint of a project is quite small (1GB).
The most straightforward way to get a file into rstudio cloud is to put it in a downloadable location (github, dropbox, etc.). The download it from that location to the project rather then trying to use the upload button here.
Sorry for the inconvenience,
Sean