These files are in HEX format. See below which is part of a bootloader file:
:02 0000 02 30 00 CC
:10 E160 00 00 47 43 43 20 56 65 72 73 69 6F 6E 20 20 3D 20 3F
Each line starts with a colon then a 2 character byte count (in hex) for the record.
This is followed an address in some records. Then a 2 character record type. Then data. Then a 2 character check-sum.
A type 02 record contains a segment address as it's data like the first line above which I have broken out. The last line is a type 00 data record. I broke this one out as well. So, each byte of data written to the flash is represented by 2 bytes in the hex file, and for every 20 of thoes there are some 12 additional characters describing what to do with the data as well as a few extra characters in each file.