Tap/click on the composer, look to the bottom right for Add Attachments. This will draw from your gallery etc. Wait for processsing, check insert full size or add manually at lrg/med/sm from below the composer once the file chooser is closed,
Mind the size of videos, long ones must be hosted elswhere and embedded.
High resolution must be smaller; and about 30 sec is max for upload on a med resolution.
