PKCS#7 is used as the format of the signature block in a signed JAR file.
PKCS#7 defines the syntax of "ContentInfo" as follows:

ContentInfo ::= SEQUENCE {
contentType ContentType,
content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL }

In our implementation of PKCS#7, the [0] tag of the "content" field is missing.
This will prevent any standard compliant implementation of PKCS#7 from parsing
the signature block file of a JAR file signed by javakey, and therefore prevent
interoperability.