Hello Elizabeth,
I routinely have students upload much larger files, so unless your internet connection is less than robust, your file size should be fine. I have noticed that uploading during off-peak hours can also make a difference now that everybody is online at the same time during the COVID19 crisis...
Other that that, I am wondering if you are using the correct upload option. I just did the following in Firefox in "student view"mode and had no trouble:
You will want to make sure you are using the "Record/Upload Media" button:

Choose the "Upload media" tab and select your file:

A blue progress bar should appear. When done, click on the yellow button that says "Post Reply".
If that does not work, you might consider posting your video elsewhere (on YouTube, or on your institution's version of YouTube?), then insert a link to your posted video into your discussion post rather than the file itself.
Hope this helps.
Good luck!
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