Constructor .FIL files
FIL files are the Archive format used in Constructor. At the beginning of the archive there is an index of all files stored. The index is encrypted. After that follows plain file data.
FIL files in the GOG version of Constructor can be accessed by copying the file "const.gog" (found in the Constructor folder) and renaming it to "const.iso". Then, mount this file and open it to find a selection of self explanatory directories each of which (except a couple) contain a "DATA.FIL" file that can be opened in FilEdit.
FilEdit 0.61 from http://www.black-mesa.freeserve.co.uk/ can be used to view, extract and create FIL archives. Source is available. All information below were taken from the source of FilEdit
0x00 uint32 num Number of Index Entries (encrypted. XOR 0x3BD7A59A)
0x04 IndexEntry[num] index The Index itself
Each IndexEntry is 17 bytes:
0x00 char[13] Filename Zero-terminated
0x0D uint32 Offset
The index is also encrypted. Pseudocode for decryption:
for each byte[n]:
byte[n] -= 39
byte[n] ^= 0xA5
byte[n] -= (27 + n)
TODO: upload python script for extraction