Allow conan_export file for Conan uploads

What does this MR do?

Conan packages are made of a collection of different files. One of which is conan_export.tgz. This file is currently not allowed as a valid filename to be uploaded using the Conan package API. This MR adds it to the list of valid files so it can be included when uploading Conan packages that make use of this file.

This MR follows what was recently done in a similar MR to allow conan_sources.tgz

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related #210550 (closed)