My GF somehow saved her 32mb .odp presentation as .odc chart

The .odc still has 32mb, but opens in Chart even after rename back to .odp.

How to restore the original presentation?

Same problem here. Did you manged to get it work?

I don’t know how but I accidentally ran into the same situation.
This steps fixed the file:

$ mkdir recovery && cd recovery
$ unzip ../yourfile.odc

Then fist edit the file META-INF/manifest.xml and change the entry for "manifest:full-path="/"" to

 <manifest:file-entry manifest:full-path="/" manifest:version="1.2" manifest:media-type="application/vnd.oasis.opendocument.presentation"/>

Second change the mimetype file to

application/vnd.oasis.opendocument.presentation

and zip it back to a new document:

zip -r ../newfile.odp *