Joplin mobile will not download any resources larger than 100MB.
This was put in place back when there was a bug causing Joplin to crash on large attachments. It's been fixed for about a year now, maybe we should get rid of this limitation, @laurent what do you think?
Thanks roman_r_m. I have been waiting for @laurent to reply hoping that he would relax this limitation. As he hasn't replied here are more findings.
It seems that although Dropbox's web view file explorer shows my 1d3907be3842bo59b6cbc423028961 file as being present Joplin Portable doesn't think it's there because subsequent syncs take a very very long time with lot's of LED twiddling on my Ethernet switch indicating that it's being transferred up there again.
To have Joplin usable again I had to delete the file via Dropbox's web view file explorer and delete it from Joplin's resource folder via Tools>Note attachments.