Added return error to fileFromReader

[rami-dabain]

Not returning an error here causes a panic when sending (Header in File object is a null map).

[wneessen]

Thanks for the PR @rami-dabain, that's a good catch.
Can you please also update the msg_test.go file to reflect these changes, otherwise the linter will fail:

msg_test.go:1708:16: Error return value of `m.AttachReader` is not checked (errcheck)
        m.AttachReader("testfile.txt", r)
                      ^
msg_test.go:1848:15: Error return value of `m.EmbedReader` is not checked (errcheck)
        m.EmbedReader("testfile.txt", r)
                     ^
msg_test.go:2850:16: Error return value of `m.AttachReader` is not checked (errcheck)
        m.AttachReader("attachment.txt", bytes.NewBufferString(ts1))
                      ^
msg_test.go:2851:15: Error return value of `m.EmbedReader` is not checked (errcheck)
        m.EmbedReader("embedded.txt", bytes.NewBufferString(ts2))

[wneessen]

Appreciated! LGTM! Thanks again for the PR and happy holidays, in case you celebrate!

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (d5b410d) 81.18% compared to head (f445a55) 75.21%.

Files	Patch %	Lines
msg.go	56.25%	5 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #161      +/-   ##
==========================================
- Coverage   81.18%   75.21%   -5.98%     
==========================================
  Files          24       24              
  Lines        2068     2074       +6     
==========================================
- Hits         1679     1560     -119     
- Misses        277      407     +130     
+ Partials      112      107       -5     

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