I had exactly the same problem after adding "apache-mime4j-0.6.1.jar" and "httpmime-4.0.1.jar" to my project. (for multipart POST'ing of binary data to my server).
I am also not familiar with pro-guard but at least I've managed to add my custom widget to the proguard.cfg - unfortunately the apache library proved even more difficult to add !
this is my fix... I have this added to the bottom of proguard.cfg :
#APACHE LIBRARY
-libraryjars /libs/apache-mime4j-0.6.1.jar
-libraryjars /libs/httpmime-4.0.1.jar
-keepnames class org.apache.** {*;}
-keep public class org.apache.** {*;}
-dontwarn org.apache.commons.logging.LogFactory
-dontwarn org.apache.http.annotation.ThreadSafe
-dontwarn org.apache.http.annotation.Immutable
-dontwarn org.apache.http.annotation.NotThreadSafe
I was hoping that the first 4 lines would be the fix. Unfortunately the dontwarn's are needed to really "fix" it.
Correct me if I am wrong (since I'm no proGuard pro), but I believe the first 4 lines make sure that the libraries are used as-is. The second section of four lines (dontwarn) just cover up the remaining crap and don't correct some underlying issue.
What the underlying issue is I don't know.