Every tag is casted to a DefineTag in flex2.compiler.swc.SwcLibrary line 208.
The GenericTag itself is not a DefineTag.

There are two possible solutions to this problem:

a) Make GenericTag extend DefineTag
b) Find out why a sequence of JPEG images is interpreted by the SWC parser as a GenericTag and what it actually should be.

Solution b would require someone of the Adobe team that is allowed to look into the file format specifications to review this. It makes more sense in my opinion.

The patch contains the updated version of GenericTag by extending DefineTag (solution a).