We have this same issue now, and what i can see is that when the owner of the site uploads (who also is a administrator) the aspx file it gets the proper string to launch the file in sharepoint: ?web=1 etc.., but if a member of the site uploads, its only a force download link for aspx files.
Dunno if this is helpful.
We tried all the different permission settings on my account, without fixing the issue for myself. But for some reason my colleauge can download my aspx file and re-upload it and it works again. Still looking for why.
