diff --git a/inc/class-tachyon.php b/inc/class-tachyon.php
index 5fa8c0d..c0fce66 100644
--- a/inc/class-tachyon.php
+++ b/inc/class-tachyon.php
@@ -111,6 +111,24 @@ public static function parse_dimensions_from_filename( $src ) {
 		return array( false, false );
 	}
 
+	/**
+	 * Get an attachment post ID by GUID.
+	 *
+	 * @param string $src The attachment source.
+	 * @return int|false
+	 */
+	public static function get_attachment_by_guid( $src ) {
+		global $wpdb;
+
+		// Trim the host name portion of the source as this could potentially have changed.
+		$src_path = wp_parse_url( $src, PHP_URL_PATH );
+
+		// Try to fetch the post ID by GUID.
+		$id = $wpdb->get_var( $wpdb->prepare( "SELECT ID FROM {$wpdb->posts} WHERE post_type = 'attachment' AND guid LIKE '%%%s' LIMIT 1;", $src_path ) );
+
+		return $id ?? false;
+	}
+
 	/**
 	 * Identify images in post content, and if images are local (uploaded to the current site), pass through Tachyon.
 	 *
@@ -192,29 +210,30 @@ public static function filter_the_content( $content ) {
 					if ( preg_match( '#height=["|\']?([\d%]+)["|\']?#i', $images['img_tag'][ $index ], $height_string ) )
 						$height = $height_string[1];
 
+					// Detect WP registered image size from HTML class
+					if ( preg_match( '#class=["|\']?[^"\']*size-([^"\'\s]+)[^"\']*["|\']?#i', $images['img_tag'][ $index ], $matches ) ) {
+						$size = array_pop( $matches );
+					}
+
 					// If image tag lacks width or height arguments, try to determine from strings WP appends to resized image filenames.
-					if ( ! $width || ! $height ) {
+					if ( ! isset( $size ) && ( ! $width || ! $height ) ) {
 						$size_from_file = static::parse_dimensions_from_filename( $src );
-						$width = $width ?: $size_from_file[0];
-						$height = $height ?: $size_from_file[1];
+						// Check if this isn't a full size URL before inferring dimensions.
+						if ( $size_from_file[0] !== false && $size_from_file[1] !== false ) {
+							$attachment_id = self::get_attachment_by_guid( $src );
+							if ( $attachment_id ) {
+								$fullsize_url = true;
+							} else {
+								$width = $size_from_file[0];
+								$height = $size_from_file[1];
+							}
+						}
 					}
 
 					// Can't pass both a relative width and height, so unset the height in favor of not breaking the horizontal layout.
 					if ( false !== strpos( $width, '%' ) && false !== strpos( $height, '%' ) )
 						$width = $height = false;
 
-					// Detect WP registered image size from HTML class
-					if ( preg_match( '#class=["|\']?[^"\']*size-([^"\'\s]+)[^"\']*["|\']?#i', $images['img_tag'][ $index ], $matches ) ) {
-						$size = array_pop( $matches );
-
-						if ( false === $width && false === $height && isset( $size ) && array_key_exists( $size, $image_sizes ) ) {
-							$size_from_wp = wp_get_attachment_image_src( $attachment_id, $size );
-							$width = $size_from_wp[1];
-							$height = $size_from_wp[2];
-							$transform = $image_sizes[ $size ]['crop'] ? 'resize' : 'fit';
-						}
-					}
-
 					// WP Attachment ID, if uploaded to this site
 					if (
 						preg_match( '#class=["|\']?[^"\']*wp-image-([\d]+)[^"\']*["|\']?#i', $images['img_tag'][ $index ], $class_attachment_id ) &&
@@ -334,15 +353,6 @@ public static function filter_the_content( $content ) {
 							$transform = 'fit';
 					}
 
-					// Detect if image source is for a custom-cropped thumbnail and prevent further URL manipulation.
-					if ( ! $fullsize_url && preg_match_all( '#-e[a-z0-9]+(-\d+x\d+)?\.(' . implode('|', self::$extensions ) . '){1}$#i', basename( $src ), $filename ) )
-						$fullsize_url = true;
-
-					// Build URL, first maybe removing WP's resized string so we pass the original image to Tachyon
-					if ( ! $fullsize_url ) {
-						$src = self::strip_image_dimensions_maybe( $src );
-					}
-
 					// Build array of Tachyon args and expose to filter before passing to Tachyon URL function
 					$args = array();
 
@@ -395,6 +405,15 @@ public static function filter_the_content( $content ) {
 						$size = [ $width, $height ];
 					}
 
+					// Detect if image source is for a custom-cropped thumbnail and prevent further URL manipulation.
+					if ( ! $fullsize_url && preg_match_all( '#-e[a-z0-9]+(-\d+x\d+)?\.(' . implode('|', self::$extensions ) . '){1}$#i', basename( $src ), $filename ) )
+						$fullsize_url = true;
+
+					// Build URL, first maybe removing WP's resized string so we pass the original image to Tachyon
+					if ( ! $fullsize_url ) {
+						$src = self::strip_image_dimensions_maybe( $src );
+					}
+
 					/**
 					 * Filter the array of Tachyon arguments added to an image when it goes through Tachyon.
 					 * By default, only includes width and height values.
@@ -826,6 +845,13 @@ public static function validate_image_url( $url ) {
 	 * @return string
 	 **/
 	protected static function strip_image_dimensions_maybe( $src ) {
+		// Check we can find the attachment by that name
+		$attachment_id = self::get_attachment_by_guid( $src );
+		if ( $attachment_id ) {
+			// Prevent stripping dimenions from src.
+			return $src;
+		}
+
 		// Build URL, first removing WP's resized string so we pass the original image to Tachyon
 		if ( preg_match( '#(-\d+x\d+)\.(' . implode( '|', self::$extensions ) . '){1}$#i', $src, $src_parts ) ) {
 			$src = str_replace( $src_parts[1], '', $src );
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 7ca76bd..d7e98f0 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -10,7 +10,7 @@
 // 2. Plugin installed inside of WordPress.org developer checkout
 // 3. Tests checked out to /tmp
 if ( false !== getenv( 'WP_DEVELOP_DIR' ) ) {
-	$test_root = getenv( 'WP_DEVELOP_DIR' ) . '/tests/phpunit';
+	$test_root = getenv( 'WP_DEVELOP_DIR' );
 } elseif ( file_exists( '../../../../tests/phpunit/includes/bootstrap.php' ) ) {
 	$test_root = '../../../../tests/phpunit';
 } elseif ( file_exists( '/tmp/wordpress-tests-lib/includes/bootstrap.php' ) ) {
diff --git a/tests/data/tachyon-1132x687.jpg b/tests/data/tachyon-1132x687.jpg
new file mode 100644
index 0000000..cddb87d
Binary files /dev/null and b/tests/data/tachyon-1132x687.jpg differ
diff --git a/tests/tests/class-resizing.php b/tests/tests/class-resizing.php
index 2e996a3..b5871bb 100644
--- a/tests/tests/class-resizing.php
+++ b/tests/tests/class-resizing.php
@@ -27,6 +27,7 @@ class Tests_Resizing extends WP_UnitTestCase {
 	 *
 	 * tachyon.jpg: 1280x719
 	 * tachyon-large.jpg: 5312x2988
+	 * tachyon-1132x687.jpg: 1280x719
 	 * Photo by Digital Buggu from Pexels
 	 * @link https://www.pexels.com/photo/0-7-rpm-171195/
 	 */
@@ -34,6 +35,9 @@ static public function wpSetUpBeforeClass( $factory ) {
 		global $_wp_additional_image_sizes;
 		self::$wp_additional_image_sizes = $_wp_additional_image_sizes;
 
+		// Ensure pre WP 5.3.1 behaviour with image file having dimensions in file name.
+		add_filter( 'wp_unique_filename', __NAMESPACE__ . '\\Tests_Resizing::unique_filename_override' );
+
 		self::setup_custom_sizes();
 
 		self::$attachment_ids['tachyon'] = $factory->attachment->create_upload_object(
@@ -43,6 +47,10 @@ static public function wpSetUpBeforeClass( $factory ) {
 		self::$attachment_ids['tachyon-large'] = $factory->attachment->create_upload_object(
 			realpath( __DIR__ . '/../data/tachyon-large.jpg')
 		);
+
+		self::$attachment_ids['tachyon-1132x687'] = $factory->attachment->create_upload_object(
+			realpath( __DIR__ . '/../data/tachyon-1132x687.jpg')
+		);
 	}
 
 	/**
@@ -72,6 +80,25 @@ public static function wpTearDownAfterClass() {
 			}
 		} );
 		rmdir( $uploads_dir );
+
+		remove_filter( 'wp_unique_filename', __NAMESPACE__ . '\\Tests_Resizing::unique_filename_override' );
+	}
+
+	/**
+	 * Prevents WP from fixing the file name during upload.
+	 *
+	 * This occurs if the file name contains dimensions as a suffix.
+	 * This is to help test for backwards compat with WP 5.3.0 and earlier.
+	 *
+	 * @param string $filename
+	 * @return string
+	 */
+	public static function unique_filename_override( $filename ) {
+		if ( strpos( $filename, 'tachyon-1132x687' ) === false ) {
+			return $filename;
+		}
+
+		return str_replace( '-1.jpg', '.jpg', $filename );
 	}
 
 	function setUp() {
@@ -411,6 +438,30 @@ function data_filtered_url() {
 				],
 				[ 1000, 1000 ],
 			],
+			[
+				'tachyon-1132x687',
+				'large',
+				[
+					'http://tachy.on/u/tachyon-1132x687.jpg?fit=1024,719',
+					'http://tachy.on/u/tachyon-1132x687.jpg?resize=1024,575',
+					'http://tachy.on/u/tachyon-1132x687.jpg?fit=1024,1024',
+					'http://tachy.on/u/tachyon-1132x687.jpg?w=1024&h=575',
+					'http://tachy.on/u/tachyon-1132x687-1.jpg?fit=1024,719',
+					'http://tachy.on/u/tachyon-1132x687-1.jpg?resize=1024,575',
+					'http://tachy.on/u/tachyon-1132x687-1.jpg?fit=1024,1024',
+					'http://tachy.on/u/tachyon-1132x687-1.jpg?w=1024&h=575',
+				],
+				[ 1024, 575 ],
+			],
+			[
+				'tachyon-1132x687',
+				'full',
+				[
+					'http://tachy.on/u/tachyon-1132x687.jpg',
+					'http://tachy.on/u/tachyon-1132x687-1.jpg',
+				],
+				[ 1280, 719 ],
+			],
 		];
 	}
 
@@ -720,6 +771,28 @@ function data_content_filtering() {
 					'http://tachy.on/u/tachyon.jpg?resize=150,150',
 				],
 			],
+			// Unknown attachment ID, unknown size, original file name contains dimensions.
+			[
+				'tachyon-1132x687',
+				'<p><img class="alignnone" src="%%BASE_URL%%/tachyon-1132x687.jpg" alt="" /></p>',
+				[
+					'http://tachy.on/u/tachyon-1132x687.jpg',
+				],
+			],
+			[
+				'tachyon-1132x687',
+				'<p><img class="alignnone" src="%%BASE_URL%%/tachyon-1132x687-150x150.jpg" alt="" width="150" height="150" /></p>',
+				[
+					'http://tachy.on/u/tachyon-1132x687.jpg?resize=150,150',
+				],
+			],
+			[
+				'tachyon-1132x687',
+				'<p><img class="alignnone" src="%%BASE_URL%%/tachyon-1132x687-150x150.jpg" alt="" /></p>',
+				[
+					'http://tachy.on/u/tachyon-1132x687.jpg?resize=150,150',
+				],
+			],
 		];
 	}
 }