Add documentation for file ContentItem
Add documentation for file ContentItem

From what I can tell "text" needs to be filename. Several (I've seen 2
so far) existing tools do this, but I don't see it documented in the IMS
spec or anywhere else.

From what I can tell we aren't using anything else for File content
items, at least for submissions (returning a File didn't seem to do
anything for RCE). I don't think "mediaType" is used for submissions; it
isn't listed in the IMS spec list of properties anyway.

refs INTEROP-7147
flag=none

Test plan:
- n/a

Change-Id: If4f281b884489ca4bd51e3670308d4f3da92e1b2
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/290791
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>
QA-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>
Product-Review: Evan Battaglia <e557ffdfc4b80aab44858030b0ae5370b86cb7b1@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms