Handle mislabeled image files better

[waisaed]

Issue:

Changes to Figure 7 of this case cannot be saved:

irb(main):016:0> pp e.errors.full_messages
["Image must be JPEG or PNG"]
=> ["Image must be JPEG or PNG"]

Image is likely a .tiff that somehow passed imagine/tiff validates method:

 message: 'cannot be larger than 2 MB' },
                    content_type: { in: %w[image/png image/jpeg],
                                    message: 'must be JPEG or PNG' }

Expected behavior:

• JPEG or PNG images can be uploaded
• other images are screened out by validation, even if mislabeled, so this state is avoided

Solution criteria:

• We can allow a wider range of formats. Uncompressed formats are non-ideal, but we already restrict image size to 2mb.
• We should safely screen out files that don't match their extension