diff --git a/CHANGELOG.md b/CHANGELOG.md
index 72447054..edda25e2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Xlsx reader crashed when reading a file with workbook protection - [#553](https://github.com/PHPOffice/PhpSpreadsheet/pull/553)
 - Cell formats with escaped spaces were causing incorrect date formatting - [#557](https://github.com/PHPOffice/PhpSpreadsheet/issues/557)
+- Could not open CSV file containing HTML fragment - [##564](https://github.com/PHPOffice/PhpSpreadsheet/issues/564)
 
 ## [1.3.1] - 2018-06-12
 
diff --git a/src/PhpSpreadsheet/Reader/Csv.php b/src/PhpSpreadsheet/Reader/Csv.php
index eaece1d2..6899773a 100644
--- a/src/PhpSpreadsheet/Reader/Csv.php
+++ b/src/PhpSpreadsheet/Reader/Csv.php
@@ -507,6 +507,12 @@ class Csv extends BaseReader
 
         fclose($this->fileHandle);
 
+        // Trust file extension if any
+        if (strtolower(pathinfo($pFilename, PATHINFO_EXTENSION)) === 'csv') {
+            return true;
+        }
+
+        // Attempt to guess mimetype
         $type = mime_content_type($pFilename);
         $supportedTypes = [
             'text/csv',
diff --git a/tests/PhpSpreadsheetTests/Reader/CsvTest.php b/tests/PhpSpreadsheetTests/Reader/CsvTest.php
index c42b6101..eeddbb08 100644
--- a/tests/PhpSpreadsheetTests/Reader/CsvTest.php
+++ b/tests/PhpSpreadsheetTests/Reader/CsvTest.php
@@ -83,6 +83,8 @@ class CsvTest extends TestCase
             [false, 'data/Reader/Xml/WithoutStyle.xml'],
             [true, 'data/Reader/CSV/enclosure.csv'],
             [true, 'data/Reader/CSV/semicolon_separated.csv'],
+            [true, 'data/Reader/CSV/contains_html.csv'],
+            [true, 'data/Reader/CSV/csv_without_extension'],
             [true, 'data/Reader/HTML/csv_with_angle_bracket.csv'],
             [true, 'data/Reader/CSV/empty.csv'],
             [true, '../samples/Reader/sampleData/example1.csv'],
diff --git a/tests/data/Reader/CSV/contains_html.csv b/tests/data/Reader/CSV/contains_html.csv
new file mode 100644
index 00000000..8f16e027
--- /dev/null
+++ b/tests/data/Reader/CSV/contains_html.csv
@@ -0,0 +1 @@
+"Column A";"<a href=''>Link</a>";"Column C"
\ No newline at end of file
diff --git a/tests/data/Reader/CSV/csv_without_extension b/tests/data/Reader/CSV/csv_without_extension
new file mode 100644
index 00000000..378dc515
--- /dev/null
+++ b/tests/data/Reader/CSV/csv_without_extension
@@ -0,0 +1,3 @@
+This;Are;Headers
+Cell A2;Number with comma;25,5
+Two colons and a comma;B|3;:,: