Fix format inference problems with IOStream

src/libmagickwand.jl

@@ -264,13 +264,29 @@ function getblob(wand::MagickWand, format::AbstractString)
 end
 
 function pingimage(wand::MagickWand, filename::AbstractString)
+ #Warning: filename is not UTF-8 on Windows
  status = ccall((:MagickPingImage, libwand), Cint, (Ptr{Cvoid}, Ptr{UInt8}), wand, filename)
  status == 0 && error(wand)
  nothing
 end
 
+#MagickSetFilename for reading, MagickSetImageFilename for writing
+function setfilename(wand::MagickWand, filename::AbstractString)
+ status = ccall((:MagickSetFilename, libwand), Cint, (Ptr{Cvoid}, Ptr{UInt8}), wand, filename)
+ status == 0 && error(wand)
+ nothing
+end
+
+function setimagefilename(wand::MagickWand, filename::AbstractString)
+ status = ccall((:MagickSetImageFilename, libwand), Cint, (Ptr{Cvoid}, Ptr{UInt8}), wand, filename)
+ status == 0 && error(wand)
+ nothing
+end
+
 function readimage(wand::MagickWand, filename::AbstractString)
  open(filename, "r") do io
+ (_, ext) = splitext(filename)
+ setfilename(wand, "a"*ext)
  readimage(wand, io)
  end
 end
@@ -289,6 +305,9 @@ end
 
 function writeimage(wand::MagickWand, filename::AbstractString)
  open(filename, "w") do io
+ #Let IM infer the format when writing to a stream
+ (_, ext) = splitext(filename)
+ setimagefilename(wand, "a"*ext)
  writeimage(wand, io)
  end
 end