Hey there, the short answer (for now) is: 100% of the opening happens in DropBox, Google Drive, Mail or wherever you’re opening the file from. You’re looking for something like “Open in...” which, in Dropbox, requiers you to follow your nose through links that say “download” and then maybe “export.” See if this helps. For consistency, you can also just mail the file to yourself from the sharing app and open it from there.
This will take some poking around but these instructions are enough to get you started. Comment here on this answer and we can move you forward after you’ve tried again. Thanks!
NOTE: According to ewb, the exact steps for Dropbox 2018-12-10 are:
Select the file, select the 3 dots ( ... ) menu, "Export", "Open In...", "Copy to MIDI Designer".