Name an image file whose path does not name itself

`getExtension()` asks `pathinfo()` and nothing else, so a path carrying no extension answers with
an empty string. `getIndexedFilename()` then builds `logo1.` -- a media part ending in a bare dot,
which the writers register under no content type at all.

The bytes know what the name does not: `getimagesizefromstring()` already backs `getMimeType()` a
few lines above, and its type constant converts straight to an extension. The lookup happens only
when `pathinfo()` came back empty and the file is there to be read, so a path with an extension
costs exactly what it did before -- no file access.

An extensionless SVG stays extensionless: it has no pixel dimensions for `getimagesize()` to
report, and there is nothing else in the file this decision could rest on.

Author: dkulyk

docs/changes/1.3.0.md

@@ -22,6 +22,7 @@
 - ODPresentation Writer : Fixed the background of a master slide being dropped on save by [@dkulyk](http://github.com/dkulyk) in [#912](https://github.com/PHPOffice/PHPPresentation/pull/912)
 - PowerPoint2007 Reader : Fixed a shape that asked for no fill being read as having no fill at all, which crashes the ODPresentation Writer on save, by [@dkulyk](http://github.com/dkulyk) in [#915](https://github.com/PHPOffice/PHPPresentation/pull/915)
 - Fixed a file with no measurable size (an SVG, a video) warning on `setPath()`, and a fresh object being looked up in the hash table under a null key, by [@dkulyk](http://github.com/dkulyk) in [#920](https://github.com/PHPOffice/PHPPresentation/pull/920)
+- Fixed the media part of an image whose path carries no extension being named with a trailing dot, and written under no content type, by [@dkulyk](http://github.com/dkulyk) in [#922](https://github.com/PHPOffice/PHPPresentation/pull/922)
 
 ## BC Breaks
 - `\PhpOffice\PhpPresentation\Slide\SlideMaster` is constructed without a background. A deck that relied on the white fill it used to write sets one with `setBackground()`, as [the documentation](https://github.com/PHPOffice/PHPPresentation/blob/develop/docs/usage/slides/introduction.md) shows.

src/PhpPresentation/Shape/Drawing/File.php

@@ -75,7 +75,15 @@ public function getContents(): string
 
     public function getExtension(): string
     {
-        return pathinfo($this->getPath(), PATHINFO_EXTENSION);
+        $extension = pathinfo($this->getPath(), PATHINFO_EXTENSION);
+        if ('' !== $extension || !CommonFile::fileExists($this->getPath())) {
+            return $extension;
+        }
+
+        // a path is free to carry no extension; the contents still say what the image is
+        $image = getimagesizefromstring(CommonFile::fileGetContents($this->getPath()));
+
+        return is_array($image) ? (string) image_type_to_extension($image[2], false) : '';
     }
 
     public function getMimeType(): string

tests/PhpPresentation/Tests/Shape/Drawing/FileTest.php

@@ -55,6 +55,21 @@ public function testPathWithoutVerifyFile(): void
         self::assertEmpty($object->getPath());
     }
 
+    public function testPathWithoutAnExtension(): void
+    {
+        $path = (string) tempnam(sys_get_temp_dir(), 'PhpPresentation');
+        copy(dirname(__DIR__, 4) . '/resources/images/PhpPresentationLogo.png', $path);
+
+        $object = new File();
+        $object->setPath($path);
+
+        // the name says nothing, so the contents have to
+        self::assertEquals('png', $object->getExtension());
+        self::assertStringEndsWith($object->getImageIndex() . '.png', $object->getIndexedFilename());
+
+        unlink($path);
+    }
+
     public function testPathWithRealFile(): void
     {
         $object = new File();