Filename length should be consistent between FPP and FileDownlink.hpp

[djbyrne17]

F´ Version	4.1.x
Affected Component	Svc/FileDownlink

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Problem Description

Svc/FileDownlink/FileDownlink.hpp line 196

#define FILE_ENTRY_FILENAME_LEN (101)

should use the FPP constant for file names, rather than having a separate (unequal) filename length.
See the comment in PR 4582