In the old Crash reporting - there is easy way to upload. i didn't find it in the Crashlytics section
UPDATE: Firebase update their docs: see here
There is no need anymore to manually upload mapping file.
1 - To preserve the info Crashlytics needs for readable crash reports, add the following lines to your config file:
-keepattributes *Annotation*
-keepattributes SourceFile,LineNumberTable
-keep public class * extends java.lang.Exception
2 - To let Crashlytics automatically upload the ProGuard or DexGuard mapping file, remove this line from the config file (app/proguard-rules.pro):
-printmapping mapping.txt
For faster builds with ProGuard, exclude Crashlytics. Add the following lines to your config file:
-keep class com.crashlytics.** { *; }
-dontwarn com.crashlytics.**
Mike from Firebase here. Crashlytics in Firebase does not offer a way to upload mapping files outside of the build flow. I'm personally curious, did you use the manual upload frequently?
If you have followed the indications to enable the automatic upload then build the app and you still see the stack traces obfuscated in Firebase you should check the log files:
On Linux / Windows: /.crashlytics/com.crashlytics.tools/crashlytics.log
On Mac: ~/Library/Caches/com.crashlytics/com.crashlytics.tools/crashlytics.log
Look for the latest entry with: Uploading deobfuscation file
And try to find the result of the operation like: Deobfuscation file(s) uploaded or Crashlytics had a problem uploading the deobs file and you will get some hints about the cause.
来源：https://stackoverflow.com/questions/47281762/how-to-upload-a-mapping-file-to-firebase-crashlytics