Check files on initial mount to confirm that the file extension matches the file type

[factus10]

Don't assume the user has correctly named the file: confirm by examining enough bytes of the file.

Is it a:

• TAP file
• TZX file
• DCK file
• BIN file

This will address David saving TZX as TAP, among other things.

[rcalcagno]

Initial support for file validation is added in the (upcoming) v1.1c, the new version will validate:

a) proper DCK image (this was already on the previous version)
b) TZX files
c) TAP files with a non-standard first block (i.e. different that 19 bytes)

Any errors are informed on the logfile.
.BIN and .ROM files are arbitrary by definition, so no validation can be done. Other checks can be incorporated as well.