Save the attachment to Files, confirm its name ends in `.ics`, then select it from inside ICSKit instead of relying on the iPhone preview.
Five reasons an ICS file may not open
1. The download is incomplete
Check the file size in Files and download it again if it appears empty. Files stored by a third-party app may still be waiting for a cloud download; tap the cloud icon before importing.
2. The extension has changed
The filename must end in `.ics`. Email services and websites sometimes deliver a text file, a ZIP archive or a filename with two extensions. Unzip it first or correct the filename if you know it is calendar data.
3. iOS shows a preview without an import action
A visible event list does not guarantee that the current preview offers “Add All.” Save the file to Files and choose it inside an importer. This is especially helpful for files with many events.
4. The calendar data has a format error
ICS uses a structured text format. A missing required field, broken line or unusual time-zone definition can stop an importer. Validate the file before repeatedly trying to add it.
5. The file is actually a subscription feed
If you received a calendar URL rather than a downloaded file, add it as a subscription in iPhone Calendar settings. Importing a static file and subscribing to an updating feed are different workflows.
Edit and validate the file on iPhone
ICSKit includes an ICS text editor and file validator. Open the saved file, inspect the validation result and correct the source only when you understand the intended event details. Keep an untouched copy so you can return to the original.
Safety first: Calendar files can contain event links and notes. Review them before importing and only use files from a source you trust.
Skip the unreliable preview
Open the saved file directly in ICSKit and diagnose it before adding events.
Try ICSKit on iPhone ↗Avoid duplicates when trying again
Before re-importing, search Calendar for one distinctive event title from the file. If a previous attempt partly succeeded, remove those events or import the retry into a separate calendar. That makes cleanup much easier.