With the update to iOS 11, images synchronised from iOS devices to a Mac are in HEIC file format. When I attempt to open these images using Affinity Photo from within Apple Photos the application/extension reports "The file type is not supported." Opening a file from Photos in the Affinity Photo standalone app is not affected as the export process changes the file to a suitable type.
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The desktop versions of Affinity Designer and Photo still cannot open HEIC/F files directly. One way you can work around this is to add the HEIC file to Apple Photos, then go to menu Image ▸ Edit With ▸ Affinity Photo (the file will be sent as a TIFF for editing in Photo) or alternatively (still in Apple Photos with the HEIF file selected) go to menu File ▸ Export ▸ Export 1 Photo... and select the format you want. You can then open the exported file dir