It is possible that you have a FlowCharter version 1 ".abc" extension file instead of a version 6 .abc file.
To explain further, a little bit of history may help:
Flowcharter version 1 = .abc
Flowcharter version 2 = .af2
Flowcharter version 3 = .af3
Flowcharter version 4 = .af3
Flowcharter version 5 (never released)
Flowcharter version 6 = .abc (The extension was recycled)
Flowcharter version 7 = .flo
Flowcharter version 8 and later = .igx (iGrafx)
When the ".abc" extension was recycled with FlowCharter version 6, the application could no longer support reading version 1 .abc files. Therefore, if you can't read a .abc file, it most likely means it is from version 1 (which is an exceptionally old format). The workaround is to use Flowcharter version 4 to read the .abc and save it out as a .af3 file, which then iGrafx should read without issue.
On a related note, the .afw file format was the old "workspace" files which allowed a user to save all the current state of their toolbars and other windows in a separate file. This data is no longer useful to move forward so we do not read any "workspace" files from previous versions, and don't support that feature any longer.
Finally, it is very possible this is not a FlowCharter file at all. The .abc extension is also associated with programming languages, audio formats, email address books, and potentially other types of files. Please ensure you have a file created by FlowCharter if the above procedures don't work, or if you continue to have problems reading the file.