Looking at the extension from comment 10, the problem seems to be that we're trying to open the file using it's URL-encoded file name, but in the ZIP-file the image is actually stored with a literal space in the file name.
Bug 1548776 Comment 11 Edit History
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Looking at the theme from comment 10, the problem seems to be that we're trying to open the file using it's URL-encoded file name, but in the ZIP-file the image is actually stored with a literal space in the file name.