Existing file with bad magic prevents save()

[dbohdan]

This is surprising behavior. The user does not expect the format of an existing file to prevent overwriting it. Here is a transcript showing the problem.

julia> using FileIO, ImageIO

julia> A = rand(10, 10)
10×10 Array{Float64,2}:
...

julia> isfile("bad.png")
false

julia> save("bad.png", A)
0

julia> open("bad.png", "w") do io write(io, "Ceci n'est pas un PNG") end
22

julia> save("bad.png", A)
ERROR: File{DataFormat{:UNKNOWN}}("/tmp/demo/bad.png") couldn't be recognized by FileIO.

Stacktrace:
 [1] save(::Formatted, ::Any; options::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /home/dbohdan/.julia/packages/FileIO/zL0JE/src/loadsave.jl:196
 [2] save at /home/dbohdan/.julia/packages/FileIO/zL0JE/src/loadsave.jl:196 [inlined]
 [3] #save#15 at /home/dbohdan/.julia/packages/FileIO/zL0JE/src/loadsave.jl:118 [inlined]
 [4] save(::String, ::Array{Float64,2}) at /home/dbohdan/.julia/packages/FileIO/zL0JE/src/loadsave.jl:118
 [5] top-level scope at REPL[16]:1

julia> rm("bad.png")

julia> save("bad.png", A)
0

julia> save("bad.png", A)
0