Hi there, @WendyApisdorf ...
I did some testing in my own sandbox course where I had an assignment that was configured to accept both a text submission and a file attachment. I submitted the assignment two different times where I had attached a file and also added some text in the text box. The submission really depends on which tab you are on and what gets submitted to your instructor. For example, if you had attached a file via the "File Upload" tab and also added text via the "Text Entry" tab, if you click the "Submit Assignment" button on the "File Upload" tab, it will only submit the file and not the text entry. Similarly, if you had attached a file to the "File Upload" tab and added text to the "Text Entry" tab but this time clicked the "Submit Assignment" button on the "Text Entry" tab, it would only submit the text entry...not the file from the "File Upload" tab.
The RCE (Rich Content Editor) can save text if you leave the page, but you'll want to read the section on "View Auto-Saved Content" near the bottom of this Guide: How do I add and modify text in the Rich Content E... - Instructure Community (canvaslms.com).
Also if you mark this as resolved, can other people still reply?
Absolutely! As you can see from this thread, you replied after Greg Beyrer's response, and I am replying to you here.
Also wouldn't marking as resolved reduce the visibility of my post? And by consequence, that potentially would take away the opportunity of someone who does have knowledge of how the Canvas system server work from replying because it's been marked as resolved?
Not at all. Everything is searchable here in the Community, so if people are looking for something specific such as this topic, they should be able to find it.
I hope this helps to answer your questions.