Sharing chat from Whatsapp with lots of media crashes K-9

I tried to share a big chat from Whatsapp (Open chat -> More -> Email chat), which caused K-9 to crash. Crash log:

D/AndroidRuntime(21342): Shutting down VM
E/AndroidRuntime(21342): FATAL EXCEPTION: main
E/AndroidRuntime(21342): Process: com.fsck.k9, PID: 21342
E/AndroidRuntime(21342): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.fsck.k9/com.fsck.k9.activity.MessageCompose}: java.util.concurrent.RejectedExecutionException: Task android.os.AsyncTask$3@adb174b rejected from java.util.concurrent.ThreadPoolExecutor@2f63f528[Running, pool size = 9, active threads = 9, queued tasks = 128, completed tasks = 0]
E/AndroidRuntime(21342):        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
E/AndroidRuntime(21342):        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
E/AndroidRuntime(21342):        at android.app.ActivityThread.access$800(ActivityThread.java:151)
E/AndroidRuntime(21342):        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
E/AndroidRuntime(21342):        at android.os.Handler.dispatchMessage(Handler.java:102)
E/AndroidRuntime(21342):        at android.os.Looper.loop(Looper.java:135)
E/AndroidRuntime(21342):        at android.app.ActivityThread.main(ActivityThread.java:5254)
E/AndroidRuntime(21342):        at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime(21342):        at java.lang.reflect.Method.invoke(Method.java:372)
E/AndroidRuntime(21342):        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
E/AndroidRuntime(21342):        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
E/AndroidRuntime(21342): Caused by: java.util.concurrent.RejectedExecutionException: Task android.os.AsyncTask$3@adb174b rejected from java.util.concurrent.ThreadPoolExecutor@2f63f528[Running, pool size = 9, active threads = 9, queued tasks = 128, completed tasks = 0]
E/AndroidRuntime(21342):        at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2011)
E/AndroidRuntime(21342):        at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:793)
E/AndroidRuntime(21342):        at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1339)
E/AndroidRuntime(21342):        at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:594)
E/AndroidRuntime(21342):        at android.content.AsyncTaskLoader.executePendingTask(AsyncTaskLoader.java:235)
E/AndroidRuntime(21342):        at android.content.AsyncTaskLoader.onForceLoad(AsyncTaskLoader.java:166)
E/AndroidRuntime(21342):        at android.content.Loader.forceLoad(Loader.java:347)
E/AndroidRuntime(21342):        at com.fsck.k9.activity.loader.AttachmentInfoLoader.onStartLoading(AttachmentInfoLoader.java:35)
E/AndroidRuntime(21342):        at android.content.Loader.startLoading(Loader.java:290)
E/AndroidRuntime(21342):        at android.app.LoaderManagerImpl$LoaderInfo.start(LoaderManager.java:280)
E/AndroidRuntime(21342):        at android.app.LoaderManagerImpl.doStart(LoaderManager.java:772)
E/AndroidRuntime(21342):        at android.app.Activity.onStart(Activity.java:1164)
E/AndroidRuntime(21342):        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1236)
E/AndroidRuntime(21342):        at android.app.Activity.performStart(Activity.java:6006)
E/AndroidRuntime(21342):        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2288)
E/AndroidRuntime(21342):        ... 10 more

Start by reproducing the WhatsApp chat-sharing flow with a chat containing lots of media and inspect the crash at AttachmentInfoLoader.onStartLoading. Trace how attachment-loading tasks are queued when MessageCompose starts. Done means sharing the large chat no longer crashes and the attachments are handled successfully.