I'm not the OP, but I also have this problem - gmail says "Failed to download the attachment. Please try again". Brand new Zenfone 8, 30.11.51.115 software, and have tried the following:
Added file/media permission to Gmail (interestingly, I was not prompted for this, I had to go to Settings->Apps->Permissions to allow it). No change.
Clearing Gmail cache & data. No change.
Removing Gmail updates (back to factory version). Clear data & cache. No change.
Made sure background data & unrestricted data usage enabled for Gmail. No change.
Made sure background data & unrestricted data usage enabled for Download manager. No change.
Of course rebooting the phone. No change.
Removed my google account from the phone and then added it back. No change.
Tried my wife's gmail account, still can't download attachments. No change.
Confirmed I can successfully download this attachment via desktop PC Gmail and also via third party Android mail apps.
I captured the logcat messages that occur when gmail tries to save the attachment, as shown below. Any suggestions???
09-20 21:06:54.104 11510 15595 E DatabaseUtils: Writing exception to parcel
09-20 21:06:54.104 11510 15595 E DatabaseUtils: java.lang.SecurityException: Unsupported path /0f87ace476e7186e844aae0c231e522d
09-20 21:06:54.104 11510 15595 E DatabaseUtils: at com.android.providers.downloads.DownloadProvider.checkFileUriDestination(DownloadProvider.java:1096)
09-20 21:06:54.104 11510 15595 E DatabaseUtils: at com.android.providers.downloads.DownloadProvider.insert(DownloadProvider.java:730)
09-20 21:06:54.104 11510 15595 E DatabaseUtils: at android.content.ContentProvider.insert(ContentProvider.java:1735)
09-20 21:06:54.104 11510 15595 E DatabaseUtils: at android.content.ContentProvider$Transport.insert(ContentProvider.java:366)
09-20 21:06:54.104 11510 15595 E DatabaseUtils: at android.content.ContentProviderNative.onTransact(ContentProviderNative.java:167)
09-20 21:06:54.104 11510 15595 E DatabaseUtils: at android.os.Binder.execTransactInternal(Binder.java:1154)
09-20 21:06:54.104 11510 15595 E DatabaseUtils: at android.os.Binder.execTransact(Binder.java:1123)
09-20 21:06:54.105 6590 6590 W Gmail : GmailAttMgr:Failed to download original file of attachment 0.1 due to java.lang.SecurityException: Unsupported path /0f87ace476e7186e844aae0c231e522d
09-20 21:06:54.105 6590 6590 E Gmail : IARH-native-sapi:Fail to download attachment for inline image.
09-20 21:06:54.105 6590 6590 E Gmail : java.lang.SecurityException: Unsupported path /0f87ace476e7186e844aae0c231e522d
09-20 21:06:54.105 6590 6590 E Gmail : at android.os.Parcel.createExceptionOrNull(Parcel.java:2373)
09-20 21:06:54.105 6590 6590 E Gmail : at android.os.Parcel.createException(Parcel.java:2357)
09-20 21:06:54.105 6590 6590 E Gmail : at android.os.Parcel.readException(Parcel.java:2340)
09-20 21:06:54.105 6590 6590 E Gmail : at android.database.DatabaseUtils.readExceptionFromParcel(DatabaseUtils.java:190)
09-20 21:06:54.105 6590 6590 E Gmail : at android.database.DatabaseUtils.readExceptionFromParcel(DatabaseUtils.java:142)
09-20 21:06:54.105 6590 6590 E Gmail : at android.content.ContentProviderProxy.insert(ContentProviderNative.java:549)
09-20 21:06:54.105 6590 6590 E Gmail : at android.content.ContentResolver.insert(ContentResolver.java:2174)
09-20 21:06:54.105 6590 6590 E Gmail : at android.content.ContentResolver.insert(ContentResolver.java:2135)
09-20 21:06:54.105 6590 6590 E Gmail : at android.app.DownloadManager.enqueue(DownloadManager.java:1119)
09-20 21:06:54.105 6590 6590 E Gmail : at lqt.f(PG:12)
09-20 21:06:54.105 6590 6590 E Gmail : at lqt.c(PG:7)
09-20 21:06:54.105 6590 6590 E Gmail : at elu.a(Unknown Source:14)
09-20 21:06:54.105 6590 6590 E Gmail : at awvc.c(PG:2)
09-20 21:06:54.105 6590 6590 E Gmail : at awve.run(PG:9)
09-20 21:06:54.105 6590 6590 E Gmail : at vie.run(PG:4)
09-20 21:06:54.105 6590 6590 E Gmail : at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
09-20 21:06:54.105 6590 6590 E Gmail : at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
09-20 21:06:54.105 6590 6590 E Gmail : at vht.run(Unknown Source:7)
09-20 21:06:54.105 6590 6590 E Gmail : at vhn.run(Unknown Source:7)
09-20 21:06:54.105 6590 6590 E Gmail : at java.lang.Thread.run(Thread.java:923)