Uploading a file to slack using webwrite

I would like to be able to upload an image to slack using file.upload API and webwrite. The API ( https://api.slack.com/methods/files.upload ) requires to pass an argument "file" of 'multipart/form-data'.

I have tried this code and small variations unsuccessfully... any suggestion?

token='mytoken';
channel='mychannel';
filename='myimage.png';
fid=fopen(filename, 'rb');
data=fread(fid, Inf, '*char')';
fclose(fid);
options = weboptions;
options.RequestMethod = 'post';
options.MediaType = 'multipart/form-data';
options.HeaderFields = string({'token', token; 'channels', channel; 'filename', filename});
response = webwrite('https://slack.com/api/file.upload', data, options);

This yields 'invalid_form_data' and no image upload.

I also tried:

token='mytoken';
channel='mychannel';
filename='myimage.png';
fid=fopen(filename, 'rb');
data=fread(fid, Inf, '*char')';
fclose(fid);
options = weboptions;
options.RequestMethod = 'post';
options.MediaType = 'multipart/form-data';
response = webwrite(sprintf('https://slack.com/api/file.upload?token=%s&channels=%s&filename=%s&file=', token, channel, filename), data, options)

with the same results

Answer 1

[Jan]

If you upload the file defining the file name, you do not have to provide the data separately. So try the secend method with providing the filename for "file=" also.

[Franco Stellari]

Unfortunately that does not work. file= has to point to the content not the filename. In curl it would be -F file=@filename and not -F file=filename.

There is not way for the webwrite function to know that it has to send the actual data in the file to the web site.

in Python it would be a request where you pass file=open(filename,'rb')

[Jan]

Unfortuantely I find only CURL examples in the documentation. Because I've never worked with slack (or CURL) yet, I cannot guess the underlying patterns for the input arguments. But you can install CURL on your computer and use system or a C-Mex function to call it from Matlab. Then the examples from the slack pages run directly.

[Juho]

Have you found solution? Since Matlab 2018, there is content provider for files which could help you:https://se.mathworks.com/help/matlab/ref/matlab.net.http.io.fileprovider-class.html?searchHighlight=matlab.net.http.io.FileProvider&s_tid=doc_srchtitle

I have similar problem when trying to upload file to Jira from Matlab. https://se.mathworks.com/matlabcentral/answers/501255-adding-attachment-to-jira-with-matlab-restful-api