@susan_oconnell @clgordie @Jen_Charles
This issue came up in the spring of last year. I believe it had to do with drag and drop from google docs for submission. There might also be an issue if the enhanced assignments (this is for students, and I think that is the name) is turned on by your school.
I will try to find the other question and edit it into this response, but in the meantime here is a work around for submitting a file if that issue comes up for a student:
1) Click on your profile on the left side and select the Files menu option
2) upload your assignment into your MyFiles folder
3) Go to the assignment page and click on submit/New attempt (new attempt if you had already submitted once before and are trying to update the file)
4) at the bottom where you upload the file , click on more options
5) select the my files folder and select your assignment that you uploaded and then click on upload, then click on submit
Ron