Return a sensible error on unknown file type

Provide a more detailed error message when the image file type cannot
be determined via magic or extension. Previously the error message said

    attempt to concatenate local 'ext' (a nil value)

3 files changed, 13 insertions, 0 deletions

diff --git a/assets/bmp-without-ext b/assets/bmp-without-ext
new file mode 100644
index 0000000..0d7f4eb
--- /dev/null
+++ b/assets/bmp-without-ext
diff --git a/init.lua b/init.lua
index 619d9b7..4d9350e 100644
--- a/init.lua
+++ b/init.lua
@@ -366,6 +366,8 @@ local function load(filename, depth, tensortype)
    local tensor
    if image.is_supported(ext) then
       tensor = filetypes[ext].loader(filename, depth, tensortype)
+   elseif not ext then
+      dok.error('unable to determine image type for file: ' .. filename, 'image.load')
    else
       dok.error('unknown image type: ' .. ext, 'image.load')
    end
diff --git a/test/test.lua b/test/test.lua
index 80299e9..0a25cd9 100644
--- a/test/test.lua
+++ b/test/test.lua
@@ -635,6 +635,17 @@ function test.test_pbmload()
 end
 
 ----------------------------------------------------------------------
+-- Load unknown image type without extension test
+--
+function test.LoadUnknownImageTypeWithoutExtension()
+  tester:assertErrorPattern(
+    function() image.load(getTestImagePath("bmp-without-ext")) end,
+    "unable to determine image type for file",
+    "unknown image type should not be loaded or unexpected error message"
+  )
+end
+
+----------------------------------------------------------------------
 -- Text drawing test
 --
 function test.test_textdraw()