After you create the record using the REST api createRecord() you should be able to get the record's ID. At this point you would need to make another REST call using either the setBinaryData() [http://documentation.progress.com/output/rb/doc/index.html#page/rb%2Fsetbinarydata.html%23] or setDataField() [http://documentation.progress.com/output/rb/doc/index.html#page/rb%2Fsetdatafield.html%23] methods.
Both methods would require the file's Base-64 encoded binary value of file, the record's id, content type, and file name at least.
I would suggest using the POST method rather than GET for file uploads as you might run into the maxHTTPParameter limit.
Hope this helps.