Better handling for screenshot grab promise

[tezoatlipoca]

receiptUpload uploads any screenshot to /fileuploadxfer and receives the complete url to the uploaded file.
since its async it returns a promise that resolves to the filename, but I could never get it to consistently resolve properly
The receiving end in receiptSend() would always get undefined.

as a workaround, receiptUpload stores the sshot file name in browser storage and receiptSend retreives and clears it from storage but that's a kludge

Need to fix this.