Incorrect File Extension Recognition Due to Double Dot Prefix (e.g., filename...mkv)

[zhouyl]

• I have checked the existing issues to avoid duplicates
• I have redacted any info hashes and content metadata from any logs or screenshots attached to this issue

Describe the bug

When a file name contains a double dot prefix before the extension (e.g., filename..mkv), Bitmagnet incorrectly recognizes the file extension as null. This issue leads to improper classification and handling of such files.

To Reproduce

Steps to reproduce the behavior:

1. Add a file with a name like filename..mkv to Bitmagnet.
2. Observe that the file extension is not recognized correctly.
3. Check the classification or handling of the file; it may be misclassified or skipped.​

Expected behavior

Files with a double dot prefix in their names should have their extensions recognized correctly, and they should be classified and handled appropriately within Bitmagnet.

Environment Information (Required)

• Bitmagnet version: [0.10.0]
• OS and version: [Ubuntu 22.04 LTS]

Additional context

This issue appears to be related to how Bitmagnet parses file names and extensions. Implementing a more robust parsing mechanism that handles double dot prefixes correctly could resolve this problem.

[rraymondgh]

Digging in a bit

This will find examples in database

WITH cte
     AS (SELECT tc.content_type,
                tf.extension,
                tf.path,
                Row_number()
                  OVER (
                    partition BY tc.content_type, tf.extension
                    ORDER BY tc.content_type, tf.extension DESC) AS row_num
         FROM   torrent_files tf,
                torrent_contents tc
         WHERE  path LIKE '%.mkv'
                AND tf.info_hash = tc.info_hash
                AND tc.content_type IS NOT NULL)
SELECT *
FROM   cte
WHERE  row_num <= 2; 

The regex that is used for parsing is:

bitmagnet/internal/model/torrent_files.go

Line 33 in 4a45981

var fileExtensionRegex = regexp.MustCompile(`[^/.]\.([a-z0-9]+)$`)

starting to put together a unit test:

package model

import (
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestFileExtensionFromPath(t *testing.T) {
	t.Parallel()

	type parseTest struct {
		inputString string
		expectedExt string
	}

	var parseTests = []parseTest{
		{
			inputString: "short film.mkv",
			expectedExt: "mkv",
		},
		{
			inputString: "short film.mkv.mk2",
			expectedExt: "mk2",
		},
		{
			inputString: "short film....mkv",
			expectedExt: "",
		},
	}

	for _, test := range parseTests {
		t.Run(test.inputString, func(t *testing.T) {
			t.Parallel()

			actualOutput := FileExtensionFromPath(test.inputString)
			assert.Equal(t, test.expectedExt, actualOutput.String)
		})
	}
}

@zhouyl - is this consistent with issue you have raised? I can continue to extend unit testing and look for a fix and possibly raise a PR.